If you're going to chop it all up into "collections," perhaps here's a reasonable solution.
1 - Music that is, in some form,
taken directly from the show, i.e. showcasing clips, remixes of songs from the show, fan covers, etc. etc.
2 - Music that is
directly inspired from the show, like fan themes for mane characters, Luna Republic, etc. etc.
3 - Music that is based on/deals with
the fandom, or tertiary characters that are only more prevalent because of the fandom ... Lyra, Derpy, and VinylScratch come to mind.
Personally, while I tend to agree with Makkon, I'm also inclined to think that we'd be depriving the studio of something for excluding a genre we're
assuming they don't like. There could be that one animator who's really into speedcore, and might really appreciate a track ending up on the compilation! In my mind, this should be the Brony Music Community's best, regardless of genre!
My 2 cents.