You need to integrate them slowly. The original flock can kill new comers. Take the new ones and keep them in a separate cage within the flocks cage or keep the new ones on the other side of the pen separated by wire. They can all see each other, but no one can touch. Keep them this way for about 2 weeks. Then release them together. There should be minimal fighting, although you will still need to watch them for a few days. If any bird is still aggressive, separate them out for a week or two, until everybody gets along. If any of them have injuries, use blu-kote or pine tar to heal and hide the wounds so no one picks them.