If the mbuna are not particularly aggressive and the peacock is aggressive or not timid, i find it works good. I have had a paired up set of albino strawberry peacocks in my mbuna tank for 6 mo now. no problems. I noticed the mbuna dwell almost exclusivly in bottom of tank and peacocks and acei hang out in the top area.
I should point out though i dont have any johanni or male kenyi in the tank anymore. alot of the more aggressive mbuna species would make it impossiable.