I would say it's better to have the pbass about 2" bigger than the oscars due to the shape of the fish. A 6" oscar is a lot bigger and bulkier than a 6" cichla.
Ok thanks for all the info guys. I am going to try this when I get my bigger tank. There will be my 2 Oscars, PeacockBass, a fire eel( to replace the one that just died), A silver Arowana, and some Ornate pims in it.