I don't have any experince with these fish.
neither do i, neither do i
. but oh well, i pretend...
back to the point, besides the food-related issue, which i think really is a non-issue, the real reason behind secluding the mbuna from the non-mbuna is the differences in behavior. the mbuna are far more aggressive and boisterous in nature than non-mbuna.
in the wild, these two groups do come in contact, but the non-mbuna have a chance of escaping into the open waters while threatened. but in the confines of the aquarium, if a red zebra decides that the lithobates is a threat to its territory, it'll chase it till it cowers in a corner, afraid to display its normal colors.
so its better not to mix too many mbuna (other than the zebra/labs/acei). from the non-mbuna group, there are certainly many other choices that you can pick out. i'd recommend having a nimbochromis sp. (venustus/livingstonii). the male looks wonderful in its breeding dress, but gorgeus otherwise too.
as regards to buying them, the cheapest and most enjoyable way is to stock 5-6 juvies of each species and them adjust the sex ratios as they grow up. a good line of food does wonders to color them up and make them grow fast. as i mentioned before, the NLS is the most-vaunted.
but keep in mind that the peacocks are the ugly ducklings in this hobby. they'll color up the last, but will the pride of your tank.