Ill probably end up doing gmixed gender as ill be buying everything from my lfs,If you're going to try all male, I would try to end up with 12, maybe 15 fish. Stick to fish that stay around 6-7", I wouldn't try venustus in a 75. They get big and pretty chunky and dont mind throwing their weight around.
Look into small haps like P. Electra, O. Lithobates, O. Tetrastigma. You can try peaceful mbuna too, like yellow labs and ps. Acei.
If you decide on mixed gender only do one people species. The females mostly look the same and the males will breed with whoever is available.