Mildly aggressive Mbuna and some Peacocks will work, but it is not a rule. As with any setup, it is best to have a contingency plan like an empty tank with a cycled filter or someone to take the offending fish off your hands.
I've kept the following mbuna with peacocks and haps in a 100 gallon without problems. But the peacocks didn't breed easily. Peacocks were Stuartgranti Maleri (pair), Stuartgranti HB red shoulder (1m/2f), Jacob (otter point) pair, 1 Eureka Red male,haps were red empress 1m/3f, pair o. lithobates, 1 m borleyi red fin, 1 m venustus, 1m/2f C. moori. If your dominant mbuna group has enough females, your mbuna should fall under his control. It worked out that my caillanos chased his 3 females mostly, and the other mbuna just enough to keep them in line, while almost never bothering the haps/peacocks, that was the red empress males job (who spends most of his time chasing his 3 females)
M. Pyrsonotos (or very similar zebra)
L. Sp. 'hongi'
all but the callainos were pretty mellow, and even then the 100 gallon provided the male enough room to be the boss.