When can I introduce younger chickens into the flock?

I have 15 birds born in early March in my main coop and run. 2 months after I got them I got another 5. They are in a smaller satellite coop without access to the big run. Currently they are about half the size of the big girls but are outgrowing their small coop. They've been able to see each other through the fence but the big ones don't pay much attention. When is it safe to move the little girls in to the big house? A few weeks ago, when they were much smaller I took one into the run and held it while the big girls gathered around. One of the sweet Buffs I have decided to take a big peck at her. Ooops! Not ready. I'm worried they'll outgrow their current confines before it's safe enough to get them integrated. Help!