From what people have said and my own personal experience, aggressive fish calm down a bit when they are in a bigger tank. How marginal that is really depends on the fish.
Please don't do it off my recommendation, as I stated before I am not very familiar with Pbass. The guide of keeping the cichlds with arowana works well. The aro is big enough to deal with the aggressive cichlids yet not big enough to eat them. Whether or not this applies to Pbass I really don't know, it was a mistake for me even to comment in this thread.