It's actually better to integrate several birds at once rather than just one bird, it is more distracting and keeps just one bird from being the "different" one.
If you can identify one or two who are the major bullies (maybe that rooster?) try pulling them out for a few days at integration. That changes the whole seniority structure top to bottom, and again leads to generalized distraction.
Personally if I had a rooster who was killing other birds, especially hens, he would be immediately dispatched.