If you raise them all together at the same siz e they should be fine. It's not like a small P Bass or Wolffish it will learn that they aren't food unless you up their aggression by live feeding. I don't understand why people treat Oscar like they are sharks if you raise it with same size fish it should be fine.
. Ok that being said, I agree with the other who replied to your post. If, after 2 years, your oscars are no more than 5" then there is definitely something wrong. They should be at least 10" if not much larger by now. The barbs should be around a foot long as well.