The crazy thing about music is you can combine two styles to make another. Kinda how Ray Charles combined Gospel and Blues to create R&B.
To me Rockabilly is just Rock, a sprinkle of Country, and a dash of Blues. Reggae is a style all on it's own. So that would be good to add. I think a good category could be Instrumental Rock. You know the stuff Vai and Satriani do. It'd be nice to learn how to compose a "Speed Metal Symphony".
Spanish Music also is big where I'm from.

What might be better is to make sub categories. Some thing like

Rock:

Rockabilly
80's Style
Classic Rock
Oldies but Goodies
Instrumental Rock
Hard Rock
Metal
etc.
etc.

More often than not you find the minor genres fall under the more general ones. You should include as many as you can even the ones that don't hold their popularity. I know there is something to learn from all music. That is why I can play country music to my friends without getting harrassed. I say include as much as space will allow. I love choices.