Yes but if the hatchery wont vaccinate, OP would have to do it themselves and
1) vaccines arent available as 1 dose - the hatchery is buying 63633736 doses at once which OP would not want to do and
2) I believe anything past day old chicks isnt effective with the Mareks vaccine
I would err on the side of assuming most small scale backyard breeders do not vaccinate their chicks, so if you intend to ever have those in your flock, I would go for a fully unvaccinated flock.
you don't have to stop medicated feed when you treat with corid IMO. it's the same medication so theres no drug interference and I doubt the increase matters very much. I dont bother with buying unmedicated chick feed for during treatment.