I have 32 12-wk old pullets and cockerels (a majority of the cockerels are Barred Rocks for meat so they won't always be around) and I finally integrated my old flock in with them about 2 weeks ago. They are: a Golden Sebright bantam cock (the leader of the pack and quite aggressive--I think he's going to meet Mr. Cone), 2 Golden Sebright bantam hens, a Mille Fleur d'Uccle cock (sweet boy), and 3 Ameraucana hens. My favorite book (The Small Scale Poultry Flock by Harvey Ussery) said to just put them in and let them work it out with supervision and a place for bullied ones to hide.
Immediately, several of the Rock cockerels circled the Sebright cock but he showed them his skillz (and his spurs) and now they leave him alone. They then began to pick on my poor little prancing d'Uccle and he ran screaming around the chicken yard, "Mon Dieu, mon Dieu!!" He wound up hiding under the roost or behind the chicken house all the time. The cockerels also picked on the smallest of the Ameraucanas but the other two girls don't take their crap and give it right back.
The Sebright girls were small enough to pop through the electric net fence each morning and head back to their old house, on the other side of our house ("Oh, there they are on their morning commute," my husband would say). It didn't work to keep putting them back so now I just let them free range around our yard during the day and sleep in their old house at night. Their old house no longer has an electric fence because I added it to the fence of the new house. Two or three days ago, Mr. d'Uccle showed up at the old house so I'm letting him stay, and I pulled the bullied Ameraucana out and put her with her small comrades because the cockerels just wouldn't leave her alone.
Of note is the fact that the girls all leave each other alone, new and old alike. Now, I don't know how it would be if there weren't cockerels and cocks to sort things out for the flock, but I bet it wouldn't be horrible and they would soon learn to live peacefully together. Yesterday was the first day I got eggs since I mixed things up: one from an Ameraucana in the new house nesting box and one from a Sebright in the old nesting box. I guess that means everyone is happy now. =)