There are two schools of thought on integrating young chicks into a flock of adults. One prefers to wait until the pullets are nearly the same size as the adults, around five or six months old, before mixing them into the flock. The other school, the one I'm in, believes in integrating chicks beginning very early, around two weeks of age. I've done this successfully with five batches of baby chicks and will be integrating a sixth batch come May.
What I do, and it requires nice weather, is to bring the babies out of their brooder to spend the warm part of the day in the run with the adults, but they are in a special "play pen", completely protected from harm, including special netting around the fencing to prevent tiny heads from getting pecked when they try to stick them through the fence.
For a couple of weeks the adults are getting used to the chicks being part of the flock, and by age four weeks, I open little 5"x7" pop holes into the main run from the chick play pen. The chicks learn very quickly to zip back into their pen when chased or pecked, and they get very good at evading the older chickens.
By age six weeks, the chicks are so much a part of the flock they are ready to move into the coop with the adults. That operation is a bit more complicated and I won't go into it right now, but this method has been very successful for me, and it gets the chicks out of their brooder in the house and moved into the coop in record time with little or no fuss.