I have never had a peacock keeps it's own with mbuna... and yes you are right wild peacocks do not normally get that big. However F1's that I like to keep are much bigger. My biggest is 6 inches and most likely done growing. I have a few smaller that are showing color as I said earlier...
I am not even sure if I would keep in a 55, if you have more than one male it will need a little more space to run if he is chased.