I just saw this post. I recently added a pink tailed chalceus to my tank as a friend for one of my other fish. I had purchased a 5" tetra of some sort, only to find out that it was very rare and are only wild caught, which means that he will not be getting any friends like him to school with. I got him a chalceus so he could have a friend that was closely related to him. It seems to be working, they started swimming together immediately. Anyway, my chalceus is about 5" inches and in the tank with a 9-10" Albino Oscar. The Oscar looked at it once and realized it was too big to eat, and he hasn't looked at it since. However, my Oscar is not really aggressive at all. I even have a few giant danios (4") in the tank, and he hasn't looked at them.
P.S. My Oscar has never had a feeder fish, so he doesn't think that every fish added to the tank is a meal. I also fed my Oscar before I added the chalceus. He didn't even notice it was in the tank for about 10 mins. Also, I have lots of hiding spots in the tank.