We've not had problems adding new chicks to our existing hens free ranging. They seem to have enough space that there is room for everyone. The younger ones seem to head to the coop earlier to get their 'spot' before the older ones come in. We have the bottom of a nesting box row that is covered so smaller ones can run under there and the bigger ones can't get in. Or, we find the smaller ones can fly better so they go up into the rafters too.
I start out by putting them in a small fenced area that is right near, but separated from my other hens. I take them out there every day starting at about 4-5 weeks. Let them run around and the other hens (and turkeys) see them and just hang out together, but not integrated. When they all seem like they are ignoring each other pretty well, I'll then one day take them out, but leave the fence open between. Usually after that, they just integrate and no problems. I always put out a second food/water location away from the main for the smaller/younger ones just in case. I have two coops also so I can use the smaller one to divide up the 'herd' if necessary, though I now have setters in that smaller coop and the bantams I had in the small coop just head for the rafters of the big coop.
Mine have about an acre of range with lots of trees, tall grass, 'hay' piles, etc. so plenty of spaces to hide and lots of stuff to keep them all busy. I also don't have any really old and established hens right now as we started over this spring with new hens, so of course YMMV with different hens.