On rereading your post, I see that was the plan - well great minds think alike. haha
I think you are worried about the mature rooster with the immature pullets. Right now, I have the same set up, and my pullets are doing just fine, but they are a sub flock within the flock, and they were raised in this flock. Mostly the older birds just ignore them. In the past, when they started laying, they become one flock.
Maybe is the answer, better odds with a few more older birds, such as a few from the other flock, but it might work, or it might not be too pretty, your older rooster should have more sense, and as long as he has a couple of other hens, I think you will be ok, but they will probably not make an established flock until the pullets start laying.
Mrs K