I kept a 4" convict with my 11" Oscar until he almost succeeded in eating it. And I also tried introducing a 6" female JD to the tank (90 gallons) and I've never seen such a scary look on my Oscar's face. He kept barrelling after her at top speed with his mouth all the way open (and it opens WIDE). Needless to say, I had to remove her to save her life.
I used to think Oscars could live peacefully with others, but definitely not mine! :-(