The real trick to keeping the peace in a community tank full of aggressive Malawian cichlids is to make darn certain that the alpha fish doesn't have an overly aggressive nature. Most Haplochromine species are unsuitable in this regard, but there are some exceptions. My personal favorite was to keep a pair of L. fuelleborni in the tank, and I mean a mature pair. A healthy male will pretty much kick the crap out of anything that challenges him, but they aren't perticularly prone to start trouble.