If your hens free range you can do what I did. About a week before I wanted to put my 11 week olds into the big girls coop I started letting them free range near the older chickens as I hovered near by to intercede if fighting broke out. Then one late afternoon/early evening I put the chicks into the coop for about an hour before the big girls returned to roost. When the big girls came in to roost the chicks were just there.
I am not saying it is completely without problems though. I wouldn't do this any younger than 11 or 12 weeks old. My nesting bucket structure is about 6" or 8" off the floor of the coop. It provides a getaway space for the chicks when the hens get too rowdy with them. So if you do introduce them like I did you should have a getaway spot for the chicks, someplace they fit that your older hens would not. Just a wooden box that will fit all the chicks for a couple of weeks that has a door small enough for only the chicks to enter would do fine. That will give the chicks the option to hide if the hens are getting rough with them. My chicks have learned when to retreat and when it's safe to come out. No serious injuries have been seen on any of the little ones. They have learned to skirt the walls when the older chickens are in the coop and during free range time they wait until the hens leave, then they come out and eat and drink for a while. When they are happily fed and watered most of them will hop out of the coop and go range away from where the big chickens are foraging.
Another option, if your coop has the room for it is to set the chicks up in their brooder, or a dog crate, inside the coop. Make sure that the big chickens can't get into it. Let the chicks live like that in the coop for a few weeks so everyone gets used to the living space sharing and then let the chicks out into the coop proper. Of course watching at first to be sure the big hens don't hurt the chicks!
