Legal army?

[Hendrik Booraem VI]

Is it legal to take a Space Marine formation as your primary detachment and an allied detachment of another army? If so, do you just nominate a character within the formation to be the warlord for your army?

I'm thinking of making an army with a 1st company task force and an allied Imperial Guard detachment, but don't want the Imperial Guard commander to be my warlord - I'd prefer a character from the Space Marines.

[Charon]

Perfectly legal.
The guard character could not even be warlord as the allied detachment may not contain your warlord.

[Hendrik Booraem VI]

Excellent, thanks. That was how I understood the rules too, but I don't play regularly, so I wanted to make sure I was understanding it right. Much appreciated!

[Blood Shadow]

This is an idea I've used previously for Death Watch, is that your intention also?

The only concern I can see you having is a single wound SM vet sgt being your warlord, you might be best adding an inquisitor or Coteaz for your Warlord