You can't pre-judge an Oscar. I had a pair that were very mellow and did everything together and never bothered other fish. Then too, I've had some that did better as a solitary inhabitant. Not matter which, they are fickle and can be moody; typical cichlid behavior. I do agree with the comment that "if it fits it's food". Currently I keep an Oscar with a 4.5" pink convict and a 15" P.disjunctivus pleco. Yes, they do squabble at times and push each other around but seem to be fairly evenly matched. With a 500 gallon tank you could afford to keep 6 and wait for a pair to match off. Puffers I can't help with.
