Usually Mike I've seen the Peacocks usually get stressed out before the Mbuna do and the Peacocks are about 2-3 times larger then the Mbuna. I don't like stress in my tanks, though, that's an almost contradiction when we say a non stressful African cichlid tank LOL. I try to minimize as much as I can. I actually found my Haps and Peacocks don't hardly go down at all, mostly mid tank, though I will say mine is a 180 and I have a pair of full grown Male Venustus too, speaking of Haps eating Mbuna....
Those 2 guys, they spend more time chasing the females then thinking to eat anyone, but you are correct. In the wild, Venustus are known to play dead to lure Mbuna fry. Maybe I've just had some sort of pixie luck, but mixing my Haps/Mbuna/Peacocks have worked out very well for the most part. Like I said, I don't want to stir up a hornet's nest being new to posting here either. The African cichlid hobby already is volitile enough as it is, I don't have any intentions to add to it LOL