Sorry to hear about your experience. I have been a nervous wreck for weeks now...here is what I have been doing and so far its worked. Fingers crossed it continues to work.
For the first time I just put my three 8 week old pullets out with my flock of three "big girls". First, I conditioned them by starting the week before. I put the three pullets inside a medium sized cage I have out in the free range area with the big girls. I kept the pullets caged all day until about 4pm with their own food and water. They loved sun bathing & being outside. I did that for about 10 days as the "introductory period". At 4pm I would put the pullets back inside their brooder.
After a week of the "introductory period" (last weekend) I let the pullets go into the free range area with the big girls & at night made them sleep in the hen house. This is how its been going...Every day they free range all together. My free range area is pretty big so they stay clear of each other. Each day for the past week just before dusk I put the pullets back in the medium sized cage, inside the run so they aren't subject to predators. My big girls have always free ranged all day and sleep inside their hen house in the coop at night. Once night fall rolls in and the big girls are in the hen house for the night, I go back out and sneak each pullet inside the hen house to sleep and be warm. Each night I hear a little rumbling around and some pecking, but after a few minutes things would settle down. Each morning around 6 or 7am I go back out and let them all out of the hen house and the run to free range again. The free range space is big enough that they stay clear of each other. On occasion I see the pullets get chased by the bigger girls, but no contact as the free range space is big enough for them to run and plenty of spots for them to hide. In addition as a distraction my hubby dumped a few cans of homemade compost in the free range area. Its full of worms, etc.. so the big girls have been digging in that all week. Good luck & hopefully this helps.