From my experience but don't get me wrong my tank size is much bigger than a 55. Is that they can live together and mine leave each other alone. My mbuna and mbuna fight, my peacocks don't mess with a thing or vise versa. My haps will chase and nip at each other Malawi and Victorian.
To answer your question on tank size. Haps all get different sizes from 4" to- -- big. So depending on the one you have it may work size wise. In a 55 your chances of higher aggression is possible. If it was me I'd take him back to see if they will exchange it being that they are mixed. Do some research before buying because even peacocks shouldn't really be mixed with some mbuna.