I'll do an OOC if there is some more interest. 

I envision a squad or so of Space Marines and a nominal leader, but we could go with small detachments of Space Marines, like a player controlling a small unit anywhere from 3-5 survivors of various engagements of the Heresy in a larger ad-hoc formation. 

They'd probably be functioning as a strike force for other loyalist elements that are not Astartes and are cut off and facing Traitors; one figures that an Astartes strike force of dubious origin is better than no Astartes strike force at all when they are on the fringes and fighting for their lives.