Do you allow requisitioning whole units to your players? Like sniper teams, sentries, sentinels, artillery (in form of barrages)? What are their rules?

I've been allowing to requisition artillery and mortar barrages. They're the same availability as ammo for them, but require Navigation Surface test to indicate the position of barrage to the artillery commander. Then he makes BS test and shoots at the target.

But I was also thinking about requisitioning of NPC's and taking them on a mission, in Mission Assigned Equipment (don't wanna invent all the availabilities for them). Especially when the task at hand is very demanding. On the battlefield they would be under squad leader's command and issuing orders would require Command test. They would receive penalties and bonuses to tests depending on whether the Command test was a success or failure.

No. But:

If they want to, I would use the usual logistic rating and put a great penatly on it, while judging the rarity from the size and activness of the warzone. So it is really hard to put a fixed bonus/penalty on it, but in general I would say: Basic infantry: Common. Specialized or elite infantry: Rare. Singe shot artillery: Rare. Regarding vehicles, at least rare, even for the most basic of bikes.

However, even all those I would only really apply in a huge warzone, where those things can be distriputed. This is not to represent the rarity of those items, but the extreme rarity of those things beeing allowed to be simply "requisitioned" by other common troopers. They need a high logistic rating to even have a chance. And then apply the usual logistic modifiers. However, just think about it: One grunt asking the boss if he can borrow other grunts. Thats not how military, let alone the Imperial guard works.

So this practice would propably be a local custom of that one regiment, making it hard to supply stuff not from the regiment. Just to give an example of the rarity: Rites of Battle for the Deathwatch RPG allows this, however, even then its costly, and the players there are Space Marine special forces. However, if those things you listed are needed or at least helpfull in the expected scenario, I would just make it mission assignment gear, so the players cant really choose, but you choose what to get. This is where those things are supposed to be.

Regarding actually using that: Sure. If the squad has at least one leader (comissar or seargent), that is buisness as usual. And the command action will be put to good use. Also remember battlefield maneuver and battlefield awarness, representing the leaders better grasp of the situation at hand. Keep however the rank, roughly represented by the squads logistic rating, in mind. See page 162 in the core rulebook, the table gives a good hint on which rating represents which influence. And if they request specialists, even low ranking ones, they are often not under direct control, say mechanicus personal, stormtroopres or scholastica psycana squads. At best, you can show them the right direction.