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.
*~Cosplays in progress~*
Sanosuke Harada - Hakuōki: Shinsengumi Kitan
Kojirō Sasaki - Iza, Shutsujin! Koi Ikusa
Kenshin Himura - Rurōni Kenshin