When we brought Huey in, our little Caique, Toby, instantly declared that Huey must die. Therefore Toby's best buddy in the world, Rio, declared that Huey must die. Huey's saving grace was that I can put him with Greycie and they somewhat get along. They're not best buddies but he can handle Greycie's ornery-ness and won't kill her. So I keep my birds in little groups and currently have three little groups that I can move around and place together.
Being a strapping male, he weighs less than Greycie by about 50 grams but he can lay the whooping on her when she needs it. She tried bullying him at first but quickly learned Huey has big muscles and can't be bossed around.