It is not uncommon for established flocks to be a bit ruthless to new comers. Pecking orders have to be rearranged when new birds come along and it can be a stressful process for all involved. For that reason a lot of people use the “see no touch” method to bring in new birds. This would be my recommendation for you to help these new ones become familiar with their surroundings and the established flock and get the established flock used to having the new ones around. It is done by providing some sort of fence or barrier between the two flocks where they can see and hear each other but they can’t interact in a way that will hurt anyone. Most do this for a few weeks and then remove the barrier. There will likely be a little bickering and some small fights and bloodshed still, but it gets everyone used to each other. Perhaps someone with a bit more experience will chime in and give you more advice about integration. I myself, have only introduced a lone bird to my flock, and I can tell you that was a really sad and hard process. Being that you have a group of new birds, the process will likely be a little bit easier; friends always help the adjustment go better. I also do not know a lot about roosters and their behavior with integration, so hoping someone can guide you on that, too.