I too have just read this post and I feel for you. Even supervised things can quickly get out of control. So my small fids have a huge cage to fly and play around in. If we let them out to play, the IRN's are locked up for that time.
Only a few weeks ago, I had two of my IRN's out supervised with our cockatiel who adores to eat with them, but we literally have to be sitting right next to the cockatiel every second, and even while we were doing this our IRN Jordan lunged and got a hold of kiely's wing. IRN's can be Very protective over their territory and their owners.
I would just take this as a sign that your IRN is a normal healthy very protective bird, he is not some crazy murderer, just a very healthy normal IRN, either protecting his territory, or himself. Love him and help him to live in a safe territory that is his. He needs to be king of his castle.
If you do have smaller fids, they are just going to have to be happy with an area all of their own, and truly they dont mind one bit.