The new girls are only six-weeks-old, and I am keeping them separated from the nine older hens until at least the end of the summer. I achieved this by fencing off part of the run with chicken wire and housing the youngsters in their own coop.
The fence allows the two groups to see each other but restricts their physical interaction. The older ladies still have access to over 400 square meters of grass in the backyard, so they have not reacted negatively to having their territory in the run encroached upon.
The new girls should be producing eggs by the end of the August. I plan to merge the two flocks after that by moving the younger hens into the main coop. If all goes well, the older girls will accept the new boarders with limited fuss and aggression.
If things don't go well, I'll probably have to contend with a gallinaceous battle royale.
We'll see . . .