I think Pinkie Pie would be a Bard. Rainbow Dash might be a Ranger, but I'm... not sure. I think Rarity would be more likely to be a Rogue than a Cleric, if only because she'd be using Profession (Seamstress) a lot, as well as going for a more skill-based character. Also, she talked her way out of the whole thing with the Diamond Dogs. Yeah, I'd say she's a Rogue.
Other than that, though, I totally agree with you on Twilight, Applejack and Fluttershy.