I would stop feeding them whatever you're feeding them right now and get them a new bag of food, (you never know), and still treat them for coccidiosis.
I didn't see anyone answer the question about how much Corid to give them.
The recommended dosage is approximately 1 teaspoon per gallon of water. Several BYC users use as much as 2 teaspoons per gallon with good success. Be sure to change the water every day
Ive had my chicks on corid for 4 days now and I bout lost them all there still dying what the heck is going on Im calling it quits on chicks till spring!
I have my doubts, just based on what you wrote about their symptoms, that is is coccidiosis. Stiff legs is not a symptom, and if they were really as skinny as i imagine from what you said, then something other than cocci is going on - possibly in addition to it, but cocci didn't cause all those symptoms, in my opinion.
I would consider getting a necropsy done if that is possible for you.