Actually, my older ones are 18
months old, not 18 weeks, so all are laying (except for my ornamental breed!). A couple of the older ones are just starting to molt for the first time so the additional protein may be good for them (read that on one of the sponsored articles on this site). I will probably just do a mixture of chick feed and layer pellets so that all feeders have the same offering and there's no fighting over the "good stuff"!
I will put out some oyster shell, as you suggest.
I put the younger ones in the coop last night with the older ones for the first time...they didn't like it at first but the older ones did leave them alone, just as you said they would. They were peeping quite a bit so I put a night light in there and that seemed to really help...I chose to use one of those battery operated candles with a timer, that are available now. It shut itself off after four hours and, by then, everyone was fast asleep. I am very cautious now about putting anything electrical near the coop...my neighbor's coop caught on fire (heat lamp) just two nights ago and all eight hens perished in the fire.
Thanks so much for your help!!!