Introducing new or just doing away with the old

Right now we have 9 hens & 1 roo that are 7 months old. We bought them from a backyard breeder who said they were australorps. Well.. they're mixed, & we really wanted full breeds. So, we ordered some full breeds, 17 pullets & 2 roos that are currently in the brooder and are about 4 weeks now. We just expanded our chicken run & have a cage inside the run for when the babies are ready to go outside, but not big enough to be put with the older ones. We figured it would be good for introducing. This cage is about 6' X 10'. I didn't realize (newbie here) until this morning that you shouldn't really put them together until the babies are about full grown at about 15 weeks. That's a long time for australorps, a bigger breed, & 19 of them to boot to be stuck in a 6' X 10' cage. We planned on selling off the older ones once the new ones started laying. Should we just avoid introducing them & sell off the new ones before moving the babies? We really didn't want to go without having eggs since we have some steady egg customers. Opinions please? What would you all do? TIA

Eggs are eggs, doesn't matter if they are purebred, so I didn't understand what you aren't keeping the older hens.

I introduce my chicks to the adults around 8 weeks. Your problem is your older ones aren't actually adults and can be unnecessarily mean to new members and it can take longer depending on their behavior. You will need to pen them next to each other for a week or two so everyone can see each other, than begin supervised mingling, rounding up the young ones when things get rough.

We plan on breeding them eventually. That's why we wanted pure breeds, not just for the eggs.

You can introduce chicks in a controlled way(separate adjacent area with small egress doors) to an adult flock.
Check this out https://www.backyardchickens.com/t/1069595/introducing-chicks-to-adults#post_16276224

Integration can be tricky but many of us do it all the time with no problems. I’ve had broody hens wean their chicks at three weeks and leave them to make their way with the flock. I regularly integrate brooder-raised chicks at 5 to 8 weeks. There are different tricks that can make this easier but two of the main keys are keeping them side by side for a while so they get used to each other and having lots of room. If your space is so tight that you basically shoehorn them together you may need to wait until they are grown to try and even them you can have problems. But if you have lots of room you can manage this at a very young age.

If you could tell us how much room you have in the coop and in the run and maybe even give us some photos we may be able to offer some specific recommendations like a safe haven, but in general give them as much room as you can, house them side by side across wire for a while, and provide different feeding and watering stations so they don’t have to compete with adults for food and water.