I did something similar. I had 3, 2 month olds that I introduced to a sole hen who was about 1 yr. I put the 3 "babies" in a cat carrier in the coop, during the day I woud let the older one out in the run and the babies I would let out in the coop. Each night I would put the little ones in the cat carrier to sleep. I did this for almost a week before letting them out together. It worked pretty well. The first day I was out in the yard to keep an eye on them and the older one did run after the babies a few times and pinned one down but I stopped her from doing any harm. It has been several weeks and they are getting along fine, although the babies and her do not socialize. They stay at one end and she at the other and if the big one comes near the babies they run to the other corner. They all go in to roost in the coop at night and the babies are getting bigger. I figure once they are her size they will establish the pecking order. I am getting chicks in a week and can't wait to do this again! Good luck!