Thanks for all the information, everybody. Like I said, this fall, I'm planning on making the existing run larger and increasing coop size to accommodate more chickens. Will building this expansion and then reintroducing all the birds (the RIRs, Orpingtons, and Millet) at once be likely to solve the aggression issue?
And about the re-integration through separation idea, the funny thing is that I let Millet roam out of her pen during the day, and she still always seems to be hanging around the pen with the other girls. When this is happening, I never see any aggression, only when she is in the pen with them is there any fighting happening.