If the chicks are healthy enough to start introducing to the older hens, the coop that you've been using for quarantine should be fine. The new chicks may introduce a new strain of coccidia in the new area, but that will come with the chicks anyway. Be ready with Corid (amprolium) to treat, if your older hens show symptoms. They may have already dealt with this strain in their lives and it won't bother them then. Treating coccidia is about building resistance in the chickens, not about eliminating it. If your older hens have had a different strain from the chicks, the chicks may need to be treated again. I know this sounds confusing and scary, but it's not bad, really.I fear it could be too full of germ, or potential coccidia oocists...