They're all different. Sometimes they accept each other with no fuss, and sometimes there's a merciless bully or two that make it their mission to torment the small ones.
I've come up with a nearly perfect solution: I install a "panic room" when introducing a new, younger crew to the older ones. Right now I have six ten-week-olds in with six one and two-year olds. I partition off half the pen with a small pop hole joining the two pens. The pass-through hole is just large enough for the smaller pullets to get through, so their feed is protected from being devoured by the big girls, and if anyone gets bullied, she can race back to the safety of the pullet pen.
For two weeks I've had my three tiny four-week-olds inside another partitioned pen inside the pullet pen. This week, I will be cutting a pop hole into that pen so the babies can begin mingling (and learning the pecking order) with the rest of the flock. Their pop holes will be just large enough for them to fit through, so neither the adults or older pullets can reach them in their panic room. As they grow, I'll need to enlarge the little pop hole to accommodate their size change.
It also helps to have two pop holes when someone is really in an urgent panic!
After everyone has achieved near same size, all the partitions can then come down. (Until you get a new crew of tiny youngsters). This gives everyone the opportunity to get to know one another, learn the pecking order without getting traumatized, and it gives me breathing room from all the refereeing, which is still a necessity at roosting time!
