To add my two cents worth...
I don't believe overstocking generally works in tanks smaller than 55 gallons. The principle behind overstocking is that aggression by one fish onto another gets dispersed: the bully starts picking on Fish A, who swims away to escape. In the process of pursuit, the bully is distracted by Fish B, goes after him, then is distracted by Fish C, etc. In my experience, smaller tanks don't have sufficient swimming room for the initial victim to get away from the bully. As a result, the bully never gets distracted and continues to pick on the original victim.
I don't doubt that some smaller tanks can find stability at high densities of mbuna, but I wouldn't want to put money on that happening often. Also, overstocked tanks require much higher maintenance and have a thinner margin of error, meaning the tank can go south quickly if something goes wrong (and Murphy's law suggests that occurence will generally be when your away on vacation).
I have to agree with those who suggest against adding more fish. I think a 29 is just too small for mbuna.
Jim