I have had Oscars for years and have learned its just best to have an other Oscar of the SAME SIZE. And this sometimes does not work but I feel its the best case scenario. Also you cannot put an African fish (demasoni) with a cichlid. I have tried Jacks several time with Oscars and in a short time it does work out because they try to kill each other. Also in your size tank with large fish such as what you are planning you can only have two Oscars and your pleco. As said, I have had Oscars with Jacks but they fight and they wound each other pretty bad when the get bigger. They also break heaters, hurt themselves on rock decor, and break tank glass. You are probably best just having one Oscar and your pleco if you want your tank to be calm or, try another Oscar of the SAME SIZE. I am not saying another fish wont work its just not likely, when you mix cichlids its a good safe bet that they will fight and fight HARD!!