If you use cBliss Tool, you can fit the number (and order) of costumes to those in Project M. While the original cBliss code forces 10 costumes for everybody, cBliss Tool handles more customizable codes. The only issue is that second team colors will not work (aka you'll have only Mario in teams, not Dr Mario too like in PM)
I tried to use the tool, but the codes didn't seem to do anything. I cycled through the costume slots and nothing was changed.