i have 8 electric yellows, 2 maingano, 1 yellow tail acei, 1 lithobate and a bgk in a 160L tank (40g) and am running a 1500Lph canister on it and i dont have a single problem with aggression. but that said imo i wouldnt go straight haps or peacock breeding setup in a 40g, rather go for mbuna and when you get a 4ft tank or bigger, the haps and peacocks will jsut outgrow it.
hth