If you ever add smaller pullets again, try to do it in units of at least three or four. They stand a much better chance of being accepted that way.
That said, I recommend the use of a panic room when adding smaller birds to the flock. Simply fence off a small corner of the run and cut small openings in each wall so the small one can slip through but not the bigger ones. Place food and water inside so the small one will always have a place to eat in peace. By the time they are too big to no longer fit through the holes in the enclosure, they should be pretty much integrated.
I have very good success integrating pullets as small as six-weeks old utilizing the panic room. You'll be amazed how quickly they figure out how to escape to safety through the small holes. You could also use a large pet crate with the door modified with an insert with a smaller access cut into it.