In general I say yah they could be kept together but blue gills and be pretty aggressive as can African cichlids so it really depends on what size tank you have, the bigger would be better so they can have their own territory. Depending on how big the blue gill is and the levels of aggression between him and the other fish it is possible he could kill them or they could kill him so add him with caution
What would you like to know about them? They are a pretty hardy fish and will eat just about anything, they can live to be about 5-8 years old but 10+ is common, they can be pretty aggressive to their own species as well as others and they grow to about 9 inches but can be seen as large as 12 foot in length, with the most growth occurring in the first 3 years.