Green terror cichlids each have their own unique personality. I’ve known some housed with Jack Dempsey, and Oscars and got along fairly well (They were raised together). And I’ve seen a green terror at my lfs when they put another similar sized one in the tank it almost immediately destroyed it. They have potential to be a very aggressive fish. If it’s been alone is whole life and is a big then I’d leave him alone in a tank. You could throw some snails in their and a few small pleco hopefully they will be fast enough and small enough he won’t bother them.