So a while back I commissioned some sprites/portraits of Mouse for an unrelated fan game that is still in development. Would I be able to use the sprite work for this, or would I need to commission new ones?  I'd assume they fall into the "other assets you have the legal rights to use" category, but I thought it'd be best to ask.

That sounds like it would fall under purchased assets - okay to use, just make sure you credit the artist(s)!