I have had to mix young pullets with mature hens twice - both times this worked with no problems at all.
I had both groups in the same open air coop (Florida style coop!) with a mesh divider between them. They could see each other through the mesh and interact a little. After a couple of days, I let them all out together - out into the "run" which is actually a large fenced veggie garden. No problems at all. A little crabbiness from the hens maybe at roosting time when a young one was encroaching on the best roosting spot, but only a little disciplining was handed out by the hens - no injuries, no bullying. I had 5 watering spots and 4 feeders for almost 50 birds, so there was opportunity for all.
Your situation is a little different because the chickens are closer in age, so the "ranks" of the individuals might be contested a little more. I actually had more of a problem with bullying among the young ones, which were only separated by a week in age, but were of several different breeds and had a big variation in size and attitude. I made sure they had a lot of space, cover for the low ranking chicks (a place to be able to rest unmolested), and things to do - like peck on plants, etc. Before I felt able to let the young ones out in the garden, I gave them big bunches of plants, cabbages cut in half, corn on the cob (stalk stuck in the wire walls), melon rinds, and so on, to keep them occupied a little.
Good luck!