Questions for you.
How long have they been on dirt?
Do you have a scale that you can weigh them on?
All young poultry and waterfowl should make daily weight gains, and any lack of weight gain or weight loss means something is wrong.
I've seen a lot of blackhead here, and that poop does not look like blackhead poop to me, to me it looks like coccidiosis or wormy poop. Any chance you can have a vet or lab test it for worms, coccidia, and blackhead?
If not, I would continue with the Corid, weigh each one, then weigh them all in a few days, and if you're on the right treatment track, those foamy yellow poops should stop and they will be gaining weight.