To address this we tried something new this year.
I let one of my Cochins set on eggs this year and put her in a fenced brooding box in the middle of my chicken run.
When the chicks hatched and grew the older chickens could see them for weeks and vice-versa. They did everything together for 7 weeks - then I let the little ones mix with the 1.5 year olds.
It was great. A little pecking from 1-2 hens for a day or two and then they were accepted.
I even mixed starter feed in with the regular layer pellets the older ones were already getting and it was a great integration.
My roosters left them alone and now 5 months later they are all one happy family.
6 Ameracana's, 4 RIR's, 2 BO's, 5 LH's, 1 Bantam Cochin, 1 Bantam Mille Fleur, 1 Grey Belgian, 1 PBR