After 4-5 days of using the corid product and seeing significant improvement, wait a week. After a week, use it for those amount of days again to be sure that no more coccidiosis parasites are laying around and straggling. By then, the parasites would've either found their way back to the chickens or not. If not, then you shouldn't worry about using it again unless you notice the symptoms once more (which would most likely be a while from their last diagnoses).
You can get more chickens after that, it is fine. The medicated food is good to keep your hens strong. It is fine to use, and more than likely to help them stay more resistant.
After the week that you start using the corid one more time to make sure the parasites are gone for good, it is 100% safe to buy more chickens.