There is nothing wrong with putting mbuna in a 30g tank or a 55g tank. If you do so, you must realize that either of these tanks will only serve as "grow out" tanks and both will eventually have to be replaced with a larger tank anyway. The absolute minimum tank size for a group of mbuna would be a 75g-90g (these tanks are 48" long and 24" front to back - the 90 is 24" tall and the 75g is 20" tall I believe). The perfect size for a mbuna tank is a 125g or a 135g, both are 60" long. This allows for an adequate rockwork layout and will provide ample area for the fish to stake out territories.
I have found in all of my years of keeping these piscavores that heavy stocking, major, major filtration, and lots of rockwork with lots of caves is the way to go. I have found that this method diminishes skirmishes and prevents losing alot of fish due to aggression.