Males can be more aggressive when females are in the tank. It's true that some males may not fully color up, but I think there will be plenty color in an all male tank if you choose your species wisely. I've seen some gorgeous Peacock tanks with all males, as well as gorgeous Mbuna male tanks. If I could setup a Mbuna tank over again, I'd do all male.
I'd skip the Pleco. Sometimes Mbuna will eat their eyes out (vulnerable part of the pleco).....so its a gamble
IMO.
If you want some catfish, get some Synodontis petricola. They need a group, so get atleast 5.