This is fine for a 55. African Cichlids usually do better if slightly crowded...as long as you don't overload the tank. I would say you have a fairly light bioload in this tank.
About the sump...no, you don't NEED one. Here are the pros and cons as I see it.
Pros: Increased water capacity. Equipment is hidden. Eg: filtration, heater, ect...
All you have in the tank is the hoses and overflow. Easier to dose additives and do water changes. You have a large filtration capacity and it's easier to service the filters.
Cons: Unless your tank is drilled, you'll have to have an external overflow to get the water out of the tank and into the sump. There is a potential for a spill with one of these. Expense. A properly set up sump, although it doesn't have to be extravagant, will cost you. You'll need the overflow, a return pump, and a suitable container to use as a sump.
