When we released both our new chickens to a pre-existing flock it wasn't pretty. The first one was only 10 weeks old (we were still fairly new to chickens and didn't know what we were doing
). Well, the chick coped pretty well, she had plenty of room in the garden and would fly up high to escape from the others when it got too much. It took maybe a few weeks to get her integrated into the flock.
Our second hen was an escaped battery hen who we found in a friend's garden. We kept her in a small cage beside the other chickens so they would get used to her, then put her in with the others but still kept them seperated with mesh. We made sure they were sharing the same water dispenser, but they still couldn't get to each other.
When we finally put them in together, there was a few days of nasty fighting and she was constantly being attacked. Sometimes i went in to help her out, but after a few days everything was back to normal and she had been accepted.
I would wait until the 10 week olds are more or less the same size as the other hen, letting her see the chicks in their cage every day, and then try introducing them together again. There will be some fighting, just be on hand to help the chicks out if it seems necessary...and if there's any blood seperate them! Hope this helps