My chicks turn four weeks tomorrow and they're mingling with the seventeen adult hens, competing for meal worms, and eating from the common feeding troughs. They also go into their coop on their own, having already mastered the plastic door flaps.
They've been with the adults since day one, separated only by a partition with small pop holes that they fit nicely through that the adults can't. There's no reason why you can't merge chicks with the adults using the "panic room" method which protects the small fry until they have the confidence to join the adults, and safety when they don't.
When the adult flock has been exposed to the tiny new members all along, by the time the chicks are bold enough to join the adults, they are already accepted members of the flock due to proximity. It's such a nifty system, it makes merging chicks with adults easy and simple.