Do the 55 African and the larger tank Central American. In my 50 gal tank I have 12 yellow labs and 12 princess with one bichir (growing out then move to big tank). Once the bichir s moved, Ill prob add 12 demosoni. Not sure on that with their agression. So far, everything is going well in the African tank. Its been running for 5 months now. I am possitive if I put them in my American tank, they would get eaten within a week. Its your money, but I wouldnt do it.
As far as the Oscar goes, I would not put it with a GT. My Stalbergi chsed it around nonstop. I eventually had to rehome the Oscar to spare its life. Now it chases the Carpintis and Jag around a lot. They are a mean fish.