If you find yourself with different ages of chicks and chickens, and you are wanting to integrate the younger into an older flock, the best way is with a transition pen. When your chicks are old enough to be outside without heat but you are worried that your older chickens will peck them too badly, if you have or can build a smaller pen inside or adjacent to your larger pen, with nothing more than screen between them, this will work wonders and save many headaches.
The transition pen should be large enough to easily handle the number of chicks that you have. Equip the pen with feeder and waterer along with a roost or two. Place your chicks in the pen and keep them there for at least 2 weeks. The larger chickens will interact with the younger ones through the fencing. They will learn each others vocalizations and motions. Over a few days time, the larger chickens will barely pay attention to the younger ones.
After 2 weeks time you should be able to release the younger ones out with the older ones for a limited amount of time. Do this at a time when you can be present for the duration of their outing. There will be some minimal pecking which is normal for establishing pecking order. Watch for aggressive behavior. Do this again on the second day for a longer duration. And by the third or forth day you should be able to leave them together for the day.
If you do witness aggressive, behavior beyond the typical pecking, it might be necessary to keep the chicks separated from the older chickens for an extra week.
This technique also works well when introducing new hens into your flock.