The MLP fighting game is still in development, but Pinkie Pie's theme has been released. IMO this would make pretty good promo music for the game.
Won't hear Pinkie Pie nor Rarity's theme until their reveal trailers are released like Twilight Sparkle's.
All of the music so far has been great. At the moment I'd say Applejack's theme is my favorite, followed by Rarity's.
I can't wait to hear what they do for Fluttershy. Fluttershy-themed fighting music seems like an oxymoron, so it's bound to be really interesting.