Just noticed the size of the tank. Might be large enough for zonatus and argentea but i would still be weary. After mine hit 6 inches or so they completely changed. I found zonatus to be slighty less aggressive than argentea but still nasty. I had to move some tanks a round and at the time some of the zo's were 4ish or less inches and i put them in a 150 with the pearsei who at the time was pushing 12ish inches. They attacked him to no end.
300 gals seem big but its really not. I feel like if i added another large male cichlid to my tank the pearsei would still find a way to have a problem with it
duanes
would be better to ask about the long term aggression issues with these fish