I suggest ya save yerself some heartache, forget the sponge filter idea, and invest in a good HOB filter. First, sponge filters only supply biological filtration and that will rapidly be minimized as the wastes and uneaten food from a tank of cichlids (which will inevitably occur unless you severely underfeed your fish) collects and clogs the pores of the sponge. Second, I really can't support lack of mechanical filtration on any cichlid tank, 'cept maybe a tank with a pair of a dwarf species. Cichlids in general are just too messy; they're messy eaters and produce significant amounts of waste. Ya'd hafta vacuum the substrate quite often to keep detritus under control; vacuuming weekly or biweekly with a water change wouldn't be enough. Third, I'm skeptical that a single sponge filter could provide enough current and surface agitation to ensure adequate gas exchange for the high bioload an overstocked tank of mbuna will place on the tank. Lack of gas exchange will inevitably lead to low dissolved oxygen. Finally, I've had mbuna pick at the sponge filters in my QT tanks, so ya need to consider the possibility the fish may gradually pick the filter apart. Sponge filters have their place in the hobby, but it isn't in a mid-sized tank full of messy cichlids.
Are ya lookin' at a species tank or a mixed tank of mbuna? I generally keep around 15 mbuna in a 55 gallon mixed species tank, with heavy filtration (500-600 gpm of flow rate through the filters). 10 fish should be alright, but just some info in case ya end up with aggression issues. Ya do have some room to play with the number of fish in the tank.
Cheers,
WYite