as the previous post above says the tank is a wee bit too small to start mixing types of mbuna but with demasoni the trick is to have a group of 10+ due to the fact demasoni are quite aggressive this will spread the aggression with overcrowding and you may get some breeding groups in that size aquarium but make sure you have excellent filtration i.e 2 filters running.
my mate has a trio in his 4ft tank with ps.flavus, crytocara moori, nimbo venustus, melanochromis auratus and an eye biter they seem to be doing fine just make sure you have enough caves or territories for them all to take hold of and you might get away with mixing.