When they go together depends.
If I put them with a broody hen, she will protect them, and they can go in with the grown hens much younger.
If there's no mother hen, then I let them get about 1/2 grown.
They are allowed to see each other, through a fence. That way, they will already know one another when the time comes for them to cohabitate.
If they are small, and without a mom, they can easily be pecked, stepped on, etc.
Also, they can be pinned in tiny areas, such as behind a feeder, where the hens lay their eggs etc., which they may not survive.
It's a jungle out there! So, in this case, I'll want them, as I said, about 1/2 grown.
I almost forgot, they eat a different food. The big birds would find a way to eat their baby crumbles, which are medicated.
At least they grow quickly!
Love All Life, Thank You For Posting! :)