The Astartes Strike Cruiser will play an interesting role since it's technically out of the Battlefleet's chain of command. The Space Marines are there (in theory) to act as senior Captain advisors and to play the critical support role of ship boarding during the void war.
I'm not going to allow Grey Knights and I'd like to discourage the use of First Founding Chapters. 40k is too great in scale imo to keep reading about the same nine loyal Chapters. And who gets what position will be first come first serve once the recruitment thread goes up.
FYI to those interested: The RP will take place in the Segmentum Pacificus in a sector of my creation (Caligo).