I keep my Oscar with 6 large ($$) Roseline sharks and 2 large SAE's. They are all 5-6 inches, and full-bodied. With that said...if he wanted to make a snack out of them....I'm sure he could. Thankfully he hasn't yet. (Knock on wood).
With that said...mine are in a 180, and I really think a 125 is bare minimum for a full sized O. When I upgraded him to a 180 from a 125....his overall behavior changed for the better. I know you may not want to hear it, but I think a larger tank may be better suited for your goals to be successful.