Introducing new chicks to an existing flock of grown birds

[WallaceFarms]

We have 5 pullets which we have had for a year and got 5 new ones in February. They are 11-12 weeks old. We have the run divided with chicken wire and 2 separate coops for now. They have been living outside in their own area of the run for 3 or 4 weeks now. We have been letting them free range together for a few hours at a time for 2 weeks. There has been some peeking order business take place but for the most part the big girls keep to themselves and the little girls stay close to their run/coop area. Today though, one of the older girls jumped on one of the younger ones and cornered her so she couldn't escspe. It's a bad deal. Young pullet has a nickel size piece of flesh and feathers missing from the back of her head. I think the bigger one would have killed her had we not been out there to stop it.
Are we going about this all wrong? What has been your experience merging new birds into an existing flock?