I'm happy to report that our integration is going well. I've really appreciated all the advice and insights in this thread.
I've been propping open the gates to the separate pens enough for the youngsters to go back and forth but not enough to be an easy fit for the older girls. The four pullets have been going out and mixing with the flock in the day, although they stick pretty close together in a clump, often running away from the adults. I've been enjoying watching them mingle and so far nobody's made too much of a ruckus.
The one that picks on them the most is Dash, the RIR who's at the bottom of the pecking order. I can't really blame her--she must see a chance to move up a notch. The little group of three hatchlings, who are now 6 weeks old have also been coming out into the big run a bit in the day to explore, although for the most part they seem quite happy in their smaller pen by themselves. I think because they were raised by a hen they have more confidence and don't seem as skittish around the bigger chickens.
Yesterday the pullet gang of four discovered the hatchlings' pen and went in there to eat their food and chase them around. The hatchlings are all big flyers though, so they just went up onto the roof of their little coop to get away.
It's interesting having the three groups at different ages. Right now the hatchlings are a few weeks younger, so they are smaller and can be picked on by the other young ones. But they're catching up fast, especially the boys, so I think the little buff bully among the pullets is going to get some comeuppance one day soon.
They're all sticking with the coops they've been raised in when they go in at night. Any tips for how/when to move the newbies into the main coop? I think they'll happily stick with their own little coops forever unless I intervene at some point.
Should I just close off their mini-coops one evening when I can be there to watch that everyone finds her way into the big coop? Or would it be better for me to deliberately move the youngsters from their coops into the big one at night when they've all settled down? At the very least, I'd like to get all the young ones into the same smaller coop.