I'd be OK with 18 hens in that set-up. They should all be able to sleep in the bigger coop without a problem, just don't leave them locked in the coop only for long periods of time.
I don't know what you plan for the future as far as replacing hens and integration, but that small coop on the far end of the run will make a great grow-out coop, a place to isolate one or more chickens if you need to, or maybe a placed for a broody if you go that route. A separate coop often comes in very handy. With that in mind I'd put a fence across the run to create a separate small run for that coop, 6' to 8' wide would be great. Put in a human door so you can get through. Just leave that door open unless you have isolated chickens using it.