I personally would not integrate until the younger ones were close to adult size. It really depends on what your setup is.....The adults will definitely bully them, and chase them away from the food and water. Plus, the larger chickens can injure or even permanently maim the younger, smaller ones.
The last time I had young chickens to add to my flock, I let them free range together, but at night, they each went back to separate coops, since I had two coops. That made it pretty easy, because the bigger chickens would chase the smaller ones, but they could hide. It took a few weeks and then everybody was getting along.