The younger chickens are 6 months old. Not laying yet - at least not finding any eggs. I originally had them isolated in the coop to introduce. They never took to perching (I had a small perch for them in their space). Now they seem to prefer snuggling together on the ground in the run. During the day I usually find them in whatever space (coop or run) that is away from the other older chickens.
To me your main issue is their maturity level. Immature pullets always rank lower in the pecking order than mature hens. If they invade the personal space of the mature hens they are likely to get pecked. It usually doesn't take them long to learn to avoid the adults day and night. It sounds like that is what yours are doing. As a general rule of thumb, most of my pullets seem to hit the maturity level where they can merge with the mature hens about the time they start to lay. This is not set in stone, some can do it earlier and some take longer. To me this is a classic case of immature pullets trying to avoid mature hens.
I personally don't believe in magic numbers, whether that is coop space, run space, roost length, or anything else. I firmly believe more room is better, if you follow the link in my signature below you'll see some of the reasons. But there are problems with magic numbers. I've seen recommendations for coop space per chicken anywhere from 1 square feet to 15 square feet. Whose numbers do you follow? The most popular on this forum is 4 square feet per chicken but several people suggest 3 is enough.
Some people suggest you need a certain amount of square feet in the run, 10 square feet per bird is a fairly common suggestion. I like run space, the more the better. But if they can't get to it then it doesn't count. I look at the coop and run as a space system. If the coop is too small they can go to the run as long as the run is available. So your management techniques in when you make the run available is an important variable. To me there are way too many variables to think that rigid numbers are required.
Typically more room is required for integration than you need after they are integrated. Especially with immature chickens they need that room to avoid the adults. And if there is conflict ("If" being an important word) they need room to run away. I would not have tried integrating those two in that tight space but you have pulled it off. If you had posted before you tried I'd have said don't do it. My definition if a successful integration is that no one gets hurt. You've done that. When they mature enough to take a spot in the mature pecking order they will join the others as one flock.
I don't know how necessary your providing heat is. To a certain point you don't need to keep the chickens warm you need to allow them keep themselves warm with their down coat. To do that they need decent ventilation to allow moisture from their breath, poop, and any thawed water to escape, but you also need to keep them out of a cold breeze. I can't tell how well your coop provides that. I don't know how well that area under the coop provides ventilation and breeze protection, you might be able to work on that. The coldest I've kept chickens was -10 F (-23 C) and some of those were sleeping in trees. Those trees were in a sheltered location so wind wasn't a big problem and you can't get better ventilation than sleeping in a tree.
One of my typical suggestions is to go by what you see instead of what somebody over the internet like me tells you that you will see. In your situation I'd keep trying to put them in the coop with the adults at night and be out there when it starts getting daylight to see how it is going. Base your actions on what you see.
I can't tell what the pop door looks like or how it is situated. If you can leave it open after they are in bed, they can leave on their own when it gets daylight so you have the benefit of the run area too when they wake up. That probably means you'll have no problems with them getting beat up.
Good luck. Usually the more room you have the easier this is. I don't know if you will be able to get them to sleep up there or not. I don't know how necessary that is.
