10-15 fish would be good. That Red Zebra is pretty aggressive and could be trouble later on. Often times, you will end up with one dominant fish and a bunch of ripped up, cowering victims. If you get a synodontis, get one as they tend to quarrel with each other. Be careful buying from mixed African tanks. The fish tend to be among the most agressive of the malawis and are almost always hybridized, making ID pretty tough. Some less aggressive Africans include the Yellow Labs(Labidochromis) and the peacocks(Auloncara). And yes, you do need a lot more rock. You want almost the whole tank full, especially if you are going with the mixed tank cichlid. Also, you may need more than a sponge filter as you increase bioload.