I'm glad they are doing better, but sorry for those you lost. It's heartbreaking, I know. :hugs Hopefully you are past the worst of it now. Make sure you complete the full treatment course, no matter how good they may look.
No need for medicated feed, the medication will be enough. Once they recover, they should have some resistance to whatever strain(s) they were exposed to. Your older birds, if they have been there for a while, should have some resistance to the strains you have. No need to treat them unless...
The correct dose of the 9.6% liquid Corid is 2 tsp per gallon of water. Make sure it's the only water they have access to, and make fresh daily, treat for 5 - 7 days. You can also give any with symptoms, or that are not drinking well on their own, an oral dose for up to 3 days, give that in...