You CAN add others, just need a bigger tank faster. I have a 75 that hosts my cichlids - Africans that is.
I would recommend not schooling, not gouramis, not danios. But other types of south Americans. Like: convicts, severums, other oscars, acards, ahlis(african), wulf cichlids(get BIG), or even some catfish like larger pictus, pangasius, or synodontis types.
In my tank I have a nice 6 inch synodontis and a clown synodontis with some clown loaches, big blood red parrot, pleco, and severum to mix with my small Africans.
Good luck!