Categories are often placed in several other categories.  See Wikipedia:Categorization for more info on categories.  As far as the convention yes, it has been what we've been doing.  It's done at every nationality's hip hop groups category.(See Category:Hip hop groups by nationality).

The reason we do it this way is so that someone can be looking at Category:Hong Kong hip hop, then head over to Category:Hong Kong hip hop musicians, and then choose between rappers and groups.  Makes navigation better.  Peace,