Ok, soume of these songs you guy probably won't have heard of. By some, I mean all. They are all J-Core songs.
Twilight Sparkle: Orion by Ryu*
http://www.youtube.com/watch?v=al_cVE7lpfA
Rainbow Dash: spEEEEEEEEEEEEEEEEdphiLiA 230pLus+ by kenta-v.ez.
http://www.youtube.com/watch?v=u52v2mTN-NI
Fluttershy: Land of Fantasy by Yoccho
only one I couldn't find a YouTube for, not one of my favourite J-Core songs because it is really slow, but I think it suits Fluttershy best.
Pinkie Pie: your rEAL by kenta-v.ez.
http://www.youtube.com/watch?v=eoqsgE3gUss
Rarity: VOX Diamond by t+pazolite
http://www.youtube.com/watch?v=xL21Z3da1h8
Applejack: Pride Or Money (Nitrogenetics Rmx) by RoughSketch
http://www.youtube.com/watch?v=6CX06FXt4yM
.
Why I chose each song:
Fluttershy: Refer to earlier in the post.
Twilight Sparkle: The song title is about space and her cutie mark is a star. She has done some space research.
Rainbow Dash: This song is very fast, abrasive, and loud, just like her.
Pinkie Pie: A hardstyle song seems like a good party-type song, don't you think?
Rarity: The song title talks about diamonds and the song talks about beautiful girls, kind of fitting, I think.
Applejack: She has a lot of pride in what she does and works really hard and likes to make lots of money.