with mbuna you can keep quite a few, i have around 25 fish in my 75 gallon, give or take a few which may be removed as they grow to keep my male to female ratios good. you could do groups of lets say 6 or 7 fish of maybe four different species. some good examples would be yellow labs, red zebras, acei, maingano or johanni. some more aggressive species would be kenyi. any of these fish would give you lots of color. and then maybe a BN pleco. the key to keeping mbuna is to overstock to avoid aggression, but you must be prepared to keep up with water changes and provide a good filtration system. im not familiar with us gallons though, canadian gallons are bigger i believe.