Sorry to say, 12-15 isn’t nearly enough in an all male 75 under 6”. One or two is almost certainly going to get singled out and killed.
25-35 is where he needs to be. That is even more important with mbuna and Tangs
Theres a difference between overstocking to manage aggression and overstocking for the sake of cramming as many fish as possible in a certain amount of space. Correct numbers will allow fish to act natural, while managing aggression.
Mbuna and tang stocking is nowhere near similar to one another.