So, some/many Rogue Traders have the power, and the resources, to support an army. If they want, they might even use their power to grab up some chunk of an Imperial Guard regiment, if they have the money, and their ship has the upgrades, to support them. Don't worry, though; this isn't one of those "how do I requisition an army?" questions. Rather, my question is "where do you find them?"

I am a huge fan of the Cadians, over the other regiments; they look like soldiers, rather than crazy Rambo rejects, gas mask fetishists, weird, fur-suit covered ice-worlders, or people with Irish prison tattoos on their faces, and they are obviously well-represented in the minis line. Originally, I sort of wanted to say "If there is a specific regiment of Guardsmen I like, how would the party recruit them ? Cadians are trained a long way from the Calixis Sector, as are the Catachans, the Kriegers, the Vostroyans, the Tanith, etc, and getting them to you, just so you can have the Guardsmen you like seems silly, and also a bit impossible. Then I thought a bit more, and said "wait! Where are you getting ANY?"

The Jericho Reach, and the Margin Crusade, just to name a few, are already gabbling up Guardsmen faster than the Nids usually do, and neither engagement can draw enough troops to end it, possibly even if the other wasn't still occurring, though neither conflict is officially in any position to be wrapped up. Having said that, where are there any Guardsmen for your crew to borrow, just so they can fly around the Koronus Expanse, spending money on their upkeep, and maybe needing them a time, or two? Even if I liked the Calixis Scintillan regiment, beyond defending the sector, and offering up troops to fight in a four-front war, or Duke Severus' traitors, how many available ones can there be, and why is whomever giving them to you, rather than sending them off to the meat grinder? Okay, I suppose I sort of am asking "how do I requisition an army?" after all, but I'm curious, even with the Rogue Trader possibly being a Trader Militant, and having the rare right to master both a ship, and an army, where are they supposed to pick one up, beyond training their own large groups of House Troops, or a PDF-equivalent, at best? Where are my Cadians, or your Fusiliers, available for me to grab them, and maybe never bring them back?

The Astra Mlitarum are a separate branch of government in the Imperium, and they are just that: a government. There are no official channels for you to request your own Imperial Guard regiment because they're controlled by a centralized government, and quite honestly they're probably all busy with war and there aren't any laying around right now. Or ever.

However it is still a political body, so maybe if you donated enough ongoing profit factor to outfit a few regiments with shiny new Leman Russes in perpetuity, then of course the regiment would need to travel with you to inspect their new gear and if for some reason they had to stop off at another planet along the way, well they would have to help you with some task because that's only fair. However I think allowing any company of Imperial soldiers to travel around with a Rogue Trader "just in case" they'll be needed is dereliction of duty, and people get shot in the head for that all the time.

I would just reskin these as mercenaries and call it a day. That's what I did.

There is always conscription.

You just find a a planet with humans, uplift them if required and then conscript them into your army. If you like the flavour of a particular type of army then you follow those practices when indoctrinating them. They might not be quite as good as the originals in the beginning but the surviving veterans will probably be quite competent after a few battles.

Bonus points if you occasionally send a few squads to the military of your choice as tithes and have them spread rumors about how working for you gets six meals a day and not just corpse starch either. Then when you need to try and requisition extra forces for whatever reason then you've got willing volunteers on the ground.

The Astra Mlitarum are a separate branch of government in the Imperium, and they are just that: a government. There are no official channels for you to request your own Imperial Guard regiment because they're controlled by a centralized government, and quite honestly they're probably all busy with war and there aren't any laying around right now. Or ever.

This is not entirely accurate. The Astra Militarum are NOT a separate branch of government. They are part of the Adeptus Administratum. Their differences of appearances and equipment have to do with two things: Their Troop type, (Armor, infantry, etc.) and their cultural point of origin. As to "Spare" regiments? I guess that's a matter of interpretation. The Administratum raises regiments as needed via the Tithe system. Simply put, they simply decide that a given world will supply x many regiments for y campaign. These regiments are normally drawn from the world's PDF. (In the case of the Cadians, all PDF's are expected to maintain an IG level of readiness for exactly this reason!)

A Rogue traders warrant often allows them to do the exact same thing! Even if it doesn't they could still theoretically "Hire" an entire regiment of troops from a PDF as mercenaries from a willing or greedy planetary governor. In fact their is at least one example in the OW fluff of a RT doing exactly that!

If the Army in question was an Imperial guard regiment and was used in a war of conquest, The RT can theoretically use them to garrison his newly won world by "Retiring" them there. He/She has the authority to do so again because of the Nature of the Warrant. Outside of Imperial space, The RT has the full authority of the council of Terra backing them!

The problem then, is not whether or not such regiments CAN be raised but rather whether the RT has the Clout or Influence to pull it off! Yes they have enough money typically to buy and outfit an entire regiment. Yes, Hives and Forgeworlds have sufficient manpower and material to provide said regiments. However, politics are a funny thing! Requests to Tithe an entire regiment to a RT would not typically be refused outright but could easily be lost in red tape literally for centuries! Thus, it becomes incumbent on the RT to Role-play (Heaven forbid!) his interaction with the governments in question! Does he roll up on Calixis and demand that governor Hax "Gimme a regiment or I start shooting!"? (That probably won't end well btw! ) or does he earnestly negotiate with the Malfian government to sweep up a few thousand troublesome gangers and "Induct" them into the Imperial guard under his command?

This is what makes RT a very different game from the other lines. The Rogue trader is a high level Peer of the imperial nobility on Par with a Space Marine Chapter master! And that's inside the Imperium! Technically he's even more powerful outside the Imperium but it's much harder to enforce. As such, A Rogue trader's calls to a planetary governor WILL be answered! It is up to your lord Captain, as a canny businessman and political opportunist, to determine where and when an opportunity exists to make the right "deal". ​​