I'd integrate them when the younger chicks are twelve weeks old or so. There are several methods to introducing new birds. Many times, its best to put the young birds in a cage/area inside of the older birds run/area. This way, they can see each other, but can't injure each other. After a week or two, let them integrate together. There will be fighting, but hopefully letting them get used to each other at first will prevent serious fighting.
You could also try introducing them at night. When they are all roosting (or sleeping, if they haven't started roosting yet), put the younger birds in with the older ones. When they wake up together in the morning, they usually won't fight as much.
No matter what you do, there will be some fighting. As long as there aren't any serious injuries, and no blood, they'll be fine eventually.
