Um.. honestly, do what makes you the happiest? You don't necessarily have to go as the "Rainbow Dash people expect".
Personally, I prefer to cosplay for myself first and foremost, rather than cosplaying for fanservice. ^^; Plus, since you're making a human/gijinka version, you have ALOT more elbow-room.
If it makes you feel any better, Rainbow Dash is usually thought to have a flat chest. Since she is athletic, it makes sense. Huge boobs would just get in the way!