1. Take the fecal sample to the vet, usually it is only $15 or so run a fecal....worth a lot of piece of mind so you know what you need to treat.
2. Corid does not always get the coccidia, and Chook is absolutely right...Sulmet needs to be given with care as it can be hard on the kidneys. A better choice, from the research I did, was sulfadimethoxine which you can get from First State Vet Supply. Follow dosage instructions carefully as this one, too, can cause damage to the kidneys if used wrong. If using sulmet or sulfadimethoxine you will need to not eat eggs for about 4 weeks after finishing treatment (I think the tx is 5 days). Retreat again 3-4 weeks later with Corid as precaution is what I did when mine got cecal coccidia. A bad bout of untreated coccidia or insufficiently treated coccidia can cause permanent damage to the intestinal system, which you would see in a necropsy. Do a google on coccidia and there is an article that shows damage of different types of coccidia to which regions of the intestines, etc. and how to identify.
3. Do some sort of probiotics either in water or yogurt/buttermilk mash to boost the good bacteria.
Good luck and I wouldn't bring any new ones in until your flock is clear for several months and/or you know what is going on.