So there are two different flocks in two different areas? Integration is best done where everyone can see each other through a fence. Every time you remove some out of sight you start all over. The older hens are defending their territory from the new invaders.They free range the entire yard during the day. When the Easter Eggers get aggressive, we separate them to the side yard which is 5' wide and 60' long.
It's always best to integrate young chicks into a flock. Older ones are harder. Size of the coop can dictate whether there's enough room for more. Submissive members need to remain at least 5 feet away, 10 is better. So in a smaller coop people will have problems because they aren't built with the rules of the pecking order in mind. Chickens need way more room than what is often recommended. Those recommendations are generally for confined debeaked birds.