Actually @azygous has a much better system and more experience with this than I do, take a look at their setup here:
https://www.backyardchickens.com/t/1069595/introducing-chicks-to-adults#post_16276224
Tho I have used the 'small door chick respite' to integrate 2 different aged chick groups and it works very well.
https://www.backyardchickens.com/t/1069595/introducing-chicks-to-adults#post_16276224
Tho I have used the 'small door chick respite' to integrate 2 different aged chick groups and it works very well.