I have a hard time imagining Rainbow Dash with (big?) boobs because, err, she's athletic. If you're comfortable or really want to add feminine curves then go for it although I personally wouldn't mind at all seeing a Dash with a "male body" because she is kinda tomboy-ish anyway.
Masamune Date - Sengoku Basara
*~Next con I'm attending~*
FIN Chibicon (9 November 2013)
SAT Ryūnosuke Ibuki - Hakuōki: Shinsengumi Kitan