I've done just as you have suggested. My coop is 12x12 and 8' high... my pen is 12x24. I've built a small 4x4 nursery (inside the coop) that opens into the pen. My thought is to let the chicks has access to the pen when they're ready, but to keep the older birds separated from the chicks. They will need to be confined to the remaining space in the coop which may be large enough but I plan to allow them outside in the evenings. Even when they are allowed outside the coop door will be open ... they will always have access to their nesting boxes. They were raised in the coop and roost there ... there is no reason to think they would ever not want to return to the coop when they are ready to go to roost.
Does that sound more reasonable now?