Well, even though VGMdb is focussed primarily on VGM, we've made a few categories that allow some anime albums to be posted, and you've described them mostly already:
- Anime based on already-established games
- Anime albums that were composed by a composer who is predominately known for composing game music
This allows the inclusion of some albums that are of interest to VGM fans, or are heavily associated with gaming and VGM. There's a lot of leeway here.
The Street Fighter anime albums you mentioned are all valid entries for VGMdb.
The Dragonball Z and Hokuto no Ken albums are not eligible, because they come from animes that really aren't based on games.
I hope this helps to explain things.