how old are all your chickens? I did this also, I bought 3 EE pullets, and when they were about 6 weeks old, I put them in a cage inside the coop with the older girls, they were 16 weeks at the time (10 weeks apart). I left them inside the coop for a few weeks so they cold all see each other...I would let the big girls out to free range all day, closing the run off, and allowing the smaller ones to 'free range' in the run. At night, I would scoop up the little ones, place them back in the cage, then let the big girls in. It was a hassle, but after a few weeks of this, I tried letting them all out together...
OOP, make sure you give the little ones some place to run and hide....they got pecked so badly, but no cuts etc...they ran away, it is hard to watch, but you have to integrate them eventually, or two separate flocks. Now at 13 weeks old, they are mingling with the older ones, but still tend to stick together, still afraid to get too close. I believe when they reach POL, the roos will make sure they are accepted, at least this is my hope...I don't want 2 flocks, I want one big happy family! LOL
Good luck, there are other ways of doing it, this is my way, and it seems to have worked.