It looks like a second player will be represented by the crew's other dancer, but is there a way to choose which dancer is representing the player if there's only one person playing? My goal is to unlock alternate costumes for the dancers, which requires you to get 25 stars with that dancer, and it appears that only stars in Crew Challenge account for that limit.
This is actually incorrect. Character selection is an option reserved for loyal dance central users. If you own the original dance central (with registration) you can use the registration card that came with the game and load all the songs from the original along with everything you have unlocked, which includes outfits and characters. Maybe the option will open after you beat the game?