Based on my O experience, he is going to be territorial no matter what fish you add and whatever will fit in to his mouth, he will try to eat. So getting a fish his size might hold its own but my O won't tolerate anyone in his tank. I've tried to put my 9" inch leporinus and O together (almost the same size) and O terrorized the lep. I moved O back to his own tank. Show your parents how cool your Oscar is: you can feed him by hand... mine will jump 3-4 inches out of the water to take the food. Out of my 9 tanks, my O is my favorite by far! Hopefully your parents will see how cool they are and drop "the other fish" stuff"... Good luck!