Adding 3 pullets to existing flock of 4. I'm at witt's end!!!

I have 4 one- year old hens. 2 welsummers and 2 columbian wyandottes. They are best buds. They are housed in a coop, ~20 sq feet, w/ a ~ 50 sq feet run, with access to a free ranging area of about 20 feet by 30 feet.

The new chicks are about 2 months old. They have been with the hens now for about a month, and it's STILL not going well. They are so mean to my new babies. They won't let them roost in the coop, even though there is plenty of space. Every night either 4 of the hens are on the roost in the coop, with the 3 chicks on the roost in the run, or vice versa. So, every night I go out and put the hens or the chicks in the coop. Much squacking is going on, with the big girls pecking the heck out of the little girls. I just don't know what to do any more. Any advice, please?

thanks gloria