That is absolutely right. You don't want to mix ages when you can help it. For some reason I had it in my head that you were going to trade in those chicks that's one of the problems with internet, I guess; losing stuff in text translation. To spot sick birds, look for discharge coming from the eyes and nose. Eyes should also be nice and bright. Check under the saddle feathers and under the wings for lice and other pests. Look at the bottoms of the feet for bumblefoot. Make sure the butt is nice an fluffy, not just for looks, but to make sure that they haven't had diarrhea. Some chickens simply can't keep their butts clean, though, and in my opinion, you don't want those. Check out the face to look for any signs of swelling or damage. If you are not confident with handling birds, you can ask the vendor to hold the bird and show you these places for you. Ask what kind of feed they have been on, if nothing more than to start conversation. They may tell you their recent illness records, or they may not. Depends on the vendor. Most return vendors should be honest in order to keep their business alive, and their customer reviews up. Sometimes you can't spot a sick bird so easily. Birds with Coccidiosis or END sometimes show no signs at all before dropping dead. I suggest reading up on chicken illnesses, if nothing more than something to fall back on once you have the birds.