Can you put up a separate cage of sorts in the coop with the older chicks? I have two sets of two - they are 9 weeks apart - and had the same issues with moving the young ones to the coop while keeping them safe from the bigger gals. I ended up putting a piece of fencing in the run to block off one end, so they could be together without the bigs having access to the littles. Each side got their own food and water. The fence had small gaps for the littles to fit through but the bigs could not, so if the littles did get brave and venture from their side of the run, they had a way back to safety. At night, I would put the littles back in their box in the garage for safe keeping. For the first week, no one cared. Second week, the bigs took notice and started chasing the littles when they got out, but never did harm. Lots of squawking, though. At night, I put the babies up in the coop and tucked them in near the big girls, and they did fine over night. I just had to be up in the morning to make sure they found safety again (they always did) That carried on for a couple more weeks but has resolved itself and they get along swimmingly.
Since your age difference isn’t as large as mine was, I suspect you’ll be fine to introduce and keep separate for a week or so and then move them in together.
