I think it all depends on how assertive the hens are. They are individuals. I have added eight new young but adult hens to a group of 12 before. There were some problems with a couple of the younger ones not being accepted by the older flock. It took some time for them to adjust but I think that was due to them not laying yet. Hens that don't lay eggs are at the bottom of the "pecking order" and they know who is laying and who is not. The picked on hens were also not assertive and didn't defend themselves.

Can you fence off a section of the hen house for the new ones so they can get to know the older ones without contact?

I've read that putting new hens in at night when everyone is asleep will help but I haven't done that. One hen got a really hard time and we had to take her out of the flock until she was laying eggs and learned to defend herself, which she eventually did. They are all one big family now, uh...usually :-)

I've introduced young birds, but I had a section of fence that they lived in when they were chicks, so the old ones could see the new ones and vice versa. I didn't let the little ones out permanently until they were big enough to fend for themselves. I would also shoo the older birds out to free range, and then close the outer fence outside the coop so the little ones could run around the hen house and yard without getting picked on. When you do let the new ones mix with the older ones, hang around for awhile to make sure the older birds don't attack the new ones. The only time I had a problem was when my one chick got out from behind the cage and almost got pecked to death! Luckily I was able to rescue him, and he survived. He remembered that I had 'saved' him and he was and still is a very social and tame rooster. He does attack my shoe in the spring though, I guess he thinks it's another rooster... :D

i tried that putting them in there at night business and they killed one of the chickens. Be careful with that. You can make a wire cage out of hardware cloth very easily and put that in the coop so the chickens can interact with each other but still be protected at the same time. That is how I plan to introduce my next set.