You cant say exactly what will work. Any given male JD with excessive aggression will try to kill all others regardless of tank size. Or claim a female, and kill the rest. I have one of those particular JD, I have tried him with a variety of big cichlids. He fights with all of them. Win or lose he goes for it. Got his but handed to him by Dovii and Red Devil. He didn't care. He lives alone, quite content that way. That said, dividing 300g into a reasonable 8 Dempseys leaves only 35ish g per fish. I wouldn't recommend any more than that. Compatibility would be the main issue I believe.