So my assumption re: Space Marines in this game is that you can't have one in the party because he would just be too powerful. I mean, aside for all the lore reasons why he wouldn't be in your party, he would also be horribly unbalanced. Right?
I barely know anything about this game, but given how there's a Space Wolf on the splash art, I think the chances are pretty good you'll get an Astartes at least temporarily.
Lore-wise it's not unreasonable for Rogue Traders to get marine "consultants" or whatever if their aims coincide.
Early-game a marine would be OP. Lategame when your characters gets a bunch of fancy stuff and are regularly counter-torturing dark eldar for giggles?
Eh, probably not so much.