I have no issues mixing vaccinated and non-vaccinated chicks as far as Marek's goes. The vaccination does not make them carriers, they are not going to infect the others. I just don't see an issue there.
@igorsMistress perhaps you could explain what issues you see?
I don't know what hatcheries Tractor Supply and that feed store are using. Different Tractor Supplies get the chicks form different hatcheries. The chicks from major hatcheries are about as safe from disease as you can get. The chickens that lay the eggs are kept separated from where the eggs are incubated and hatched. They are professionals and have very high biosecurity standards. From that aspect I would not worry as long as the feed store chicks are coming from an established hatchery.
Each TSC and feed store has different people working there. TSC has some rules to try to enhance biosecurity but some employees might be better at following those than others. Rules like customers can't just pick up chicks and put them back. I don't know how that feed store is managed. To me the biosecurity practiced by that specific TSC and feed store would be the weakest link. Unless I saw something that raised a flag I would not feel uncomfortable in getting chicks from both. To me it would be safer than getting chicks form swaps, auctions, bringing home a chicken from a chicken show, or getting chicks from a neighbor. Nothing in life is without risk but that's pretty low level risk.
