Here's how I integrate baby chicks, and I certainly don't wait until they're full grown. That just creates more problems.
I've found that the best way to integrate chicks is to brood them alongside the adult flock. That way, everyone knows one another from day one.
If you can't brood chicks in the same coop or run where your adult chickens are, the next best thing is to bring the chicks outside to a safe pen as soon as weather permits. In Florida and California, weather is permissive of this most all year.
The chicks begin with short outings in a safe pen that protects tiny heads from poking through and getting pecked by curious adults. They can stay out as long as it's warm and they don't get chilled. My chicks are brooded in the run under a heating pad system, so they have ready access to heat as needed.
Regardless of whether your chicks are brooded in the run as mine are or spending part of the day in the run, they need to be at least two weeks old and having observed the adults for at least one week before they are allowed to mingle with them.
Chicks learn by watching the adults in action. They are fast studies. They quickly learn the individual temperaments, and this serves to let them know which adults they can relax when near and which ones they need to flee.
When it's time to mingle, I open the portals from their safe pen. I call it the panic room because they learn quickly to run back inside when harassed. The more entrances into this panic room, the safer the chicks will be. You want to eliminate all dead ends and places where chicks can be cornered by an adult bully. I cut portals into every vertical barrier in the run so a chick running along it can quickly find a safe outlet. The secret is these portals are approximately 5 x 7 inches and only chicks can fit through them. You may need to make them smaller if you have any petite bullies. You can also create horizontal accesses along the panic room. For example, if you use a dog kennel as a safe pen, you can raise it up a few inches for the chicks to scoot under.
This is getting too long, so read my article on outdoor brooding for more information and photos.
