ok then yeah you are lucky its working for you, i def wouldnt add anything except maybe silver dollars or something, but i def wouldnt add any cichlids you dont want to throw off the balance, the only times i have had psycho kill-offs in the past was when i got the bright idea to change fish around and sell some and add new ones and then there was chaos as they rearranged the hierarchy, hopefully it all stays peaceful just be sure to move them all at the same time