The secret of merging young pullets with the older flock is to give them a place where they can retreat to safety. I call it the "panic room", and it's allowed me to merge six-week olds with adults with very little problem.
All you need is a smaller enclosure within the run. I usually partition off a 3'x4' corner of the pen. I start bringing the babies out to spend the day in the pediatric enclosure at around four weeks. After a week or two, I cut small openings into the partitions leading into the larger pen so the tykes can begin to explore and learn to mix with the older girls. They learn very quickly to scoot back into their safe haven when chased and pecked. Their food and water is also inside their safe area, so they are never prevented from getting enough food.
I've also merged six-week olds into the coop. They learn to run out of the coop in the mornings and make a bee-line right for their panic room. At night, they may need assistance for a week or so to go into the coop at night, but I encountered very few problems since the older girls were usually more focused on roosting than harassing chicks. As they mature, they will spend less and less time inside their little pen. By the time they're almost the same size as the adults, it's time to take down the small enclosure.
This panic room method has worked wonderfully well for raising and merging five batches of chicks.