I felt a bit sheepish after I read MissPrissy's post, so did some research here. Miss P has a WHOLE lot more experience than I have! I've had chickens for a year, and have raised flocks in the past as well, but nowhere near her experience.
Fully feathered is supposed to be 8-12 weeks, but leghorns are described (Henderson) as "very early maturing," so yours should be very close to fully feathered, if not so. Sounds like you do not know the actual date you bought them.
I didn't realize some feeds are medicated with antibiotics rather than amprolium. I wouldn't feed one with an antibiotic, for sure, but Miss P and I do go different directions on the amprolium, partly because of my climate and soil type, and that this is farm country with neighboring flocks. Just one of those things one has to check out and make one's own decision.
I also realized you did not actually say you had seen bloody poops; that was my misreading. I have read, though, that they can have an overload of cocci without actually seeing blood.
So now I am wondering: Did you get to talk with the vet about what was happening, and the vet recommended the Sulmet?