At first I added 4 at once, all about 3". Later on I was able to add one by itself every once in a while without any aggression but I'm not sure how I pulled it off. Lots of hiding places and well fed happy fish I guess lol. I did have some very mild bullying at one point when I added a smaller discus to my tank, but turning the lights off and temporarily moving some decor around helped. I also had added a new plant which I think distracted the bigger guys and I had 3 rope fish that acted as peacekeepers, it was hilarious!
Sorry I know this is an old thread lol