IMO malawis shouldn't be kept in anything smaller than a 4ft tank, the only exception to that I think acceptable is a breeding colony of p.demasoni in a 3ft.
When I first stared keeping mbuna I had a 3ft tank with l.caeruleus (one of the most peaceful mbuna), I originally started with 5, but the male killed off all but two females. I had a couple of successful broods, but as soon as the juveniles started reaching sexual maturity, the original male and both females would kill them off. Trust me, it doesn't work long term.