Since you have a Green Terror, you will need to put other South American cichlids. You could add an Oscar, Jack Dempsey, Convict, Blue Acara, other other aggressive fish. Don't add African cichlids since they won't tolerate each other.
Try to add fish that are around the same size as the Green Terror so that you don't have any fish getting picked on.
You should also add some large rocks, plants, etc. to give the fish places to hide. I used to raise cichlids. They will literally kill each other if they don't have places to hide.
Good Luck
