when and how to introduce hens to new chicks

I would guess they will always be dominate....the pecking order. However you should be able to get them all to at least fight fare or tolerate each other. Keep introducing them and you will get there. A couple things that worked for me was putting the new chicks in with the old after dark.

Also taking one old bird at a time and putting them in with all the chicks for a little bit seemed to help.

once I built a new coop. I moved young and old birds in there together after proper introduction at each other's coop. The new environment was enough distraction that all the birds got a long as if they grew up together.

 

The best way to integrate new chicks with adult chickens is to brood the chicks alongside the adults so the adults accept them as flock members from the beginning. Consider that when planning your next batch of baby chicks.

So, now you need to play catch-up by getting the chicks out with the adult hens in a safe enclosure so they will be safe from attack during the introduction phase.

The best and quickest way to do this is by putting up a temporary "panic room" where you have small chick-size openings that are too small for the adults. Keep the openings closed for a week during the introduction phase, then open them, allowing the chicks access to the run or outside. The chicks can duck back inside whenever the going gets rough, and food and water left inside for the chicks can't be taken over by the big chickens.

For pictures of this set-up in my run go down to the second link under "Articles" and read the one on outdoor brooding.