Thank you, that is a wealth of info about worms and testing! I will see if our state vet diagnostics lab will do this for us, in case I find another odd-looking poop. Might also call up one of our local vets although I have a feeling they may say no if they find out the poop is from a chicken. There are three vets in our rural area and none of them will work with birds. Sadly, in this part of the country they are seen by many as disposable.
To answer your question about feed, Scratch & Peck is a grain mix that also contains vitamins, minerals and supplements in the form of powder and small particles. The corn is included in the cold weather version of this feed that our feed store carries fall through spring; in the summer they switch to a corn-free version. I ferment it and feed 2-3 times a day, as much as they will eat (usually add a little extra for the last feeding of the day to ensure everyone gets enough, so there is often a small amount of uneaten food left when they go to roost). Spring through fall, our chickens also free range on a little under half an acre of lawn with a mix of grasses, alfalfa, clover, dandelion, etc.
Six of our hens, including the one with the bald butt, were with a different family until a couple of months ago, and were fed Nutrena pellets and a wheat and corn scratch. That family gave us their leftover pellets and scratch when they gave us the hens, and we gradually transitioned them from that feed to fermented Scratch & Peck.