They're going to need places to get out of the line of sight of the older hens; places that they can get to, but the big ones cannot. Crates, buckets laying on their sides, logs, construction blocks, a strategically placed sheet of plywood or metal roofing material, a old tire, etc. all work well for places for chicks to duck into, under, around or behind. Food & water should be in the "safe" for chick areas that the adult birds are incapable of accessing to ensure that the grown birds cannot deny groceries to the younger ones. If both groups of birds can see and hear each other all day, every day, then the little ones should be able to integrate when they're about 6 weeks of age, weather permitting. Upon integration, you will want for the entire flock to be eating an "all flock" feed with oyster shell, eggshell or calcium supplement served on the side for the adult birds to consume as needed. Layer feed has too much calcium for non-laying birds and is detrimental to the development of younger birds, and overall health of older, non-laying birds. Adding one roosting bar about six inches from the floor and not directly under the roosting bar for your adult birds will go a long way towards training your little fluffers to roost. You can raise this bar little by little until they're big enough, and friendly enough, with the older birds. This early training goes a very long way in discouraging sleeping in nesting boxes later. Be advised that integrated flocks tend to become friendly/friendlier with each other, but often do not form the same close bonds with the other group of birds as they will with their brooder-mates.
I do not recommend introducing chicks at a younger age to the group unless they are being raised by their mother and that their mother is already an established member of the flock. I consider five weeks of age an absolute minimum age to introduce chicks that are not accompanied with their mother. The risk of death due to the flock killing them for motherless chicks with adult birds is high. The risk of death for the chicks who have a mother that is an established member of the flock is exceptionally low, their mother will guard the chicks well.