A 120 would work with an Oscar. I agree with the above. I would let the Oscar put on good size before adding a con. They pack a punch and a full grown make still might jack an Oscar up.
If you are buying them all as juveniles, it could work. There is no exact since on what will and will not work. However experience may tend to favor an outcome one way more than another way. But if they are juveniles and it is something that you really want to try and pay close attention to, then go for it.
I agree with Smitty, if you add them all together as juvies, let them grow up together, there is a chance they may set up indivdual areas, and learn to accept them in a sort of detente.
If at some point you decide to add another cichlid though, this may upset the entire hierarchy, and lead to chaos.
Of course if any reach maturity before the others, this can change the dynamic over night.