I free range, so my results may not be typical. I placed my chicks outside in a brooder next to my coop at age 1 wk. This brooder had a window into the big hen's coop, separated by wire.
At 4 wks they were free ranged with the bigger gals. No problems there, as they each had their own feeders and waterers in their respective pens and seemed to favor these...for 2 wks.
Then I took away the younger chicks feeder and opened the window leading from the big house into the little pen, shut their outside access to their pen. This way they had to go through the big house to get to their little pen. They still sleep in there but they eat and drink with the big gals.
Soon I will close off their pen until they get used to sleeping with the big gals. Then I'll open up access to the other pen for all chickens and they can flow back and forth, if they please. I don't think they will get the hang of roosting unless they watch the big girls in action.
So, first, visual contact only. Free range contact next. Then, eating and drinking from same source. Next will be sleeping together. This is the worst scenario for little ones, as the pecking order for roosting seems the most stringent for these big gals. They are now at 7 wks and are over half the size of the older hens! They are quicker and fleet of foot, so they can pretty well hold their own in a chase and peck situation.
I don't know if things would have gone so smoothly if these birds had been confined to a coop and run situation.