In the current campaign which I am GMing, I have come accross the situation of having to handle requisitioning forces. Sounds simple enough, except for the slight detail of the inquisitor our acolytes work for being dead. Long story short, our inquisitor was betrayed by his right hand man, a powerful psyker, and the party has decided to stick together to get revenge. Everything is going pretty well and everyone is having fun. To not totally mess up the party, I said that the inquisitors ship and some of its crew have stuck around, and are willing to help them out (they also want revenge). A fair number of the crew and acolytes died with the inquisitor, or have just left, so right now the ship is pretty undermaned, but still works.

Our parties psyker got the cool idea of trying to recrew the ship, and has even gone through the work of coming up with a decent and pretty cool way of getting our hands on enough gelt to pay the salaries. I have figured out what most things would cost, and which ones would be paid for by the inquisition (it is after all an inquisition ship, so it does have some sway) and which ones we would have to pay for. The problem I am having is when the idea of getting some members of the imperial guard to join our crew.

With the party in a guardsman (commander) who has recently acquired a decent promotion. The idea that has been thrown around is getting her to use her position to get some guards to come and join us. It is a cool idea I want to let happen. As a commander in the imperial guard, getting troops under our command does not seem impossible, but taking them off and keeping them is a little bit more complicated.

My questions are this:
1: Would a commander in the guard be able to take soldiers without having to flash the inquisitorial seal/bribe someone/or some other form of major action to get access to the soldiers.
2: If they are taken, who pays for them? Would we have to take on that burden, or would the imperial guard continue to pay them, or some other option

3: How often would imperial guards even get paid? They are often deployed for months/years at a time, so I can't think they would be holding all their money with them.

2: If they are taken, who pays for them? Would we have to take on that burden, or would the imperial guard continue to pay them, or some other option

3: How often would imperial guards even get paid? They are often deployed for months/years at a time, so I can't think they would be holding all their money with them.
I always wondered about the way salary works, but there's no concrete "canon" answer (there rarely is lol) so we have to use our imagination. There IS a paymaster of the Imperial Guard, it's in some chain of command trees that I can find in the future. The paymaster is at the highest levels, and since there is no unified currency of the Imperium I have always imagined it to be given a kind of scrip, "IOU" from the Imperium to the government of the planet, or lump payments in land after a disbandment, etc. Gelt is exclusive to the Calaxis Sector and I wouldn't imagine a distant bureaucracy paying in Gelt directly.
I remember a funny in-universe book about the Departmento Munitorium. Basically, when IG units are stationed on a friendly planet, it is that governor's duty to provide for them (at least the basics, like food). After all, they are being provided for the benefit of the planet. If the planet is hostile, or hesitant to pay for room and board, well then the IG can take it by force.
For off duty R&R, I could see IG troopers handing "legal tender" scrip (that is printed by the quartermaster general or whatever) to disappointed shopkeepers. Eventually, that scrip can be traded in for the local currency. Maybe on worlds with big garrisons, that scrip can even be a parallel currency, kinda like the USD in Mexico.
Now if it was a regular Inquisitor requisitioning them, I'd say yeah, the Dep. Munitorium would be worrying about the scrip. Since it's more low-level unofficial stuff I'd just roleplay it depending on the number of troops. A whole regiment would probably have scribes accounting for all the scrip they are printing out, so they`d just keep printing and sending the receipts to the higher ups. A company wouldn't have that capacity, but the regiment CO could be persuaded to keep them on the books for the adventure, since it's "only" a hundred men, and they could use some seasoning.* Your PCs could be either given the scrip in advance, or the guardsmen could be given a lump sum on their return.
*fun idea, maybe have the colonel clean house and pawn off the worst troops in the regiment.

My two shells:

1: Would a commander in the guard be able to take soldiers without having to flash the inquisitorial seal/bribe someone/or some other form of major action to get access to the soldiers.

Not officially. The military is networked, and just because someone has a high rank does not give them unlimited authority, given how this person should have their own role and position where they are actually expected to lead. In fact, this officer showing up to order other troops around could throw up all sorts of unwelcome questions. Assuming her credentials check out, why is she here? What position is she assigned to as per her papers? Can Segmentum Command confirm her posting? Imperial Guard personnel doesn't just travel around the galaxy solo, and someone like that showing up would be rather unusual indeed.

That being said, there are some important considerations possibly alleviating this issue:

• Influence : Your Guardsman might be able to procure sufficient documentation to authorise her mission and reinforce her words. This would be a bit tricky as it'd involve working various contacts in the Munitorum's chain of command, but it'd be a very helpful prep-work should someone in the garrison attempt to verify her claims. It would be an excellent opportunity to make use of an Acolyte's web of contacts, provided the player actually bothered to build suitable connections.
  
  Alternatively, they could fall back on Inquisitorial authority to acquire green light without the soldiers in question being aware of it, simply because Segmentum Command will keep it a secret. Remember that Inquisitorial authority is rather fluid, and your Inquisitor does not need to be alive in order to make use of it (albeit their Influence would probably decline the longer he/she is "MIA", unless they take steps to alleviate this effect).
  
• Fellowship : The Köpenick-effect , where the officer is being obeyed just because every soldier in the regiment is trained to follow orders. This won't work in every regiment and highly depends on local culture, but the more disciplined and "robotic" the soldiers are (think DKoK), the more likely they'll just follow her orders as it would be entirely inconceivable that she might not be who she says she is.
  
• Timing : The soldiers in question might be in a situation where nobody has the time to actually verify the character's claims. The regiment might be active in a warzone, possibly having lost a portion of its officers. In this case, most troopers would probably fall back on obeying the orders of any officer who shows up, provided your player's Guardsman manages to gel with the troops rather than coming off as a foreign object with nothing but a fancy uniform to their name. The soldiers' willingness to submit to her command would likely depend on regimental culture, as well as the psychological condition of the soldiers (think Apocalypse Now).

Like Newfish, I am not of the opinion that Guardsmen actually get "paid". It is their holy duty to fight in the Emperor's wars, and the Imperium does not even have a uniform currency to pay these troops that get shipped all over space. The Imperial Navy has even been known to press-gang random civilians into duty by landing parties knocking them out and dragging them to their ship, and some Imperial Guard regiments like the Savlar Chem-Dogs are made up of condemned criminals.

What they have a right on, however, is proper billeting, food and medical care -- they fight for the Imperium, and the Imperium provides for their basic needs. So, these things should obviously be taken care of.

It should also be noted that Guard regiments seem to lack the logistics-heavy infrastructure we see in the modern military. Given how codex descriptions mention stuff like civilian wagon trains and slave labour, I would compare them more to the armies of the Napoleonic age, which means they'd simply go to some farmstead and "requisition" (aka plunder) the food they need. So depending on where your commander deploys these troops, this could be an easy way to resolve the food situation. Needless to say, this strategy is better suited for frontier worlds where such actions would raise little suspicion from sector authorities; a marauding Guard regiment might invite swift investigation by the Adeptus Arbites or even another Inquisitor, unless the Acolytes have taken steps to make sure that any news won't reach their intended destination.

The aforementioned Imperial Scrip is also something I have added to my 40k headcanon, as I first saw this idea in one of James Swallow's novels and, upon investigating the real world precedent , it makes perfect sense for the conditions and the economy of the Imperium of Man. If you are curious, I go into this idea including usage of scrip in some more detail here .

Hope that helps, and good luck!

I would say the best and fastest way for them to get some Guardsmen would be to forge a munitorium order moving the unit under their command, obviously if this came to light you might be in stum (particularly if how it came to light was this world fell to chaos because the guard weren't here to keep stuff under control).

It might be easier to requisition PDF troopers or to just buy a load of cheap guns and exploding collars and ask the arbrites if they have any criminal scum they'd like to get rid of......

My view on money and the imperial guard is that many regiments get paid in a munitorium scrip which is basically a promise that the local planetary government will pay the bearer x amount of thrones which the planetary government gets off of its tithe. I'm thinking that some regiments probably don't get it because of their status (penal legions for example) and others get swindled out of it by their officers or the admin guys (particularly feudal and so on, I imagine officers might get the money and then be told to distribute it among their unit, pay based on the number reported to be there, I imagine some units will be dangerously understrength so they can skim the pay). I've always had it that some things can only be bought using actual money so there is a thriving business of buying scrip for less then its value, basically a way of guardsmen getting around the authorities trying to limit their access to drink drugs and those with negotiable affections.