I have 3 older hens, and 3 younger hens. When the little ones were big enough, I let them free range together, although the little ones did get chased once in a while. As the little ones got bigger, and closer to the same size, I took the leap and stuck them in the big girl coop one night. One out of the three adapted well, the other two needed a little bit more time. So I gave it another month of free ranging together, but separate quarters at night, then I tried it again, and Voila! It worked!
Now they are all together, and nobody is getting picked on, but I can definitely tell what the pecking order is. It takes some time, some finesse, but it will work out sooner or later.