Might want to try this: set up a temporary run space outside for both groups of birds to use, big as you can manage. Stakes with some snow fencing, deer fencing, chicken wire, whatever you've got or can get for cheap. Because it's not predator proof you'll need to keep an eye on them, but it'll be a bit safer than having them completely free ranging. Clutter it up with whatever you have handy (like my shed has plenty of junk like old pallets, broken lawn carts, etc), then let both groups have access to it. Hopefully they will see it as neutral space - territory that belongs to neither side - and with the clutter they'll be busy exploring instead of picking at each other. I would try doing that for a good week or so, for a few hours at a time (whatever you have time for), and hopefully that will familiarize them with one another enough that when they're back in their normal enclosure, the "other birds" won't seem as foreign anymore.
As far as using the coop, don't force the younger ones to join the older ones. When they can mostly get along outside of the coop, you can start encouraging them to use the coop by shutting off access to wherever they're currently spending the night. OR... if you plan on building the new coop very very soon, you may want to hold off on trying to get them to coop together and do it at that time. Just as with the temporary run suggestion, the new coop would become neutral ground to both groups, so you're less likely to see bullying going on as the chickens will be too busy trying to figure out what's going on in this new environment.
As far as using the coop, don't force the younger ones to join the older ones. When they can mostly get along outside of the coop, you can start encouraging them to use the coop by shutting off access to wherever they're currently spending the night. OR... if you plan on building the new coop very very soon, you may want to hold off on trying to get them to coop together and do it at that time. Just as with the temporary run suggestion, the new coop would become neutral ground to both groups, so you're less likely to see bullying going on as the chickens will be too busy trying to figure out what's going on in this new environment.