All male is fine for the peacocks provided you do not have any fish that look the same color.
With mbuna, it can work as all male but it is best if they are in harems of yes, 1 Male to 2-3 Female.
Assuming the tank is 55g then as I said 12 fish max for either the peacocks or the mbuna. If it is any other size then it drastically changes what you could potentially stock.
Is the tank bowfront or rectangular?
One way of finding out the size would be to get the dimensions of the tank. (Length, Width and Height).
Mbuna females will have color whereas peacock females will not. Both males will be colorful.
Feeding would be high quality flakes and pellets with the occasional treat of a few things...dependent on what you stock.