Pwny may suggest a small modification? Although you do not, I know many players play multiple characters and there is no reason to take that tool away from a player's skillset unless necessary. I think people should be able to play whatever character they want at the time of their match, they just cannot change that character between matches.
Also, here is the standard stage PM ruleset which have been slightly altered to fit the schema of a crew battle:
Project M Stage Picking:
1. For game 1 of the crew battle, one team will select from one of the bottom row (neutral) stages.
(I'd suggest that the team that picks the very first stage of the crew battle was somehow at a disadvantage during the team making process. For example, one captain may pick first, while the other gets pick of first stage)
2. Subsequently, all stages on page 1 are legal counterpicks against the winner of the previous game.
3. Winner of previous match CANNOT switch characters between matches.