I would not add terramycin- it has no effect on Coccidia. I would feed them yogurt to replenish their gut flora and fauna and give the Amprolium at full dose for a full 5 days, being careful to keep them in a very clean brooder and not allow them to eat poop. Sulfa has an effect on coccidia even though it is not meant for protozoa, as it interrupts the life cycle of the oocysts. It is MUCH harder on the gut due to the antibiotic/antimicrobial qualities.
Terramycin will kill bacteria, but in doing so, it will kill of any remaining beneficial bacteria, but will have no effect on the protozoa.
For today, give them yogurt and feed them normally, and tomorrow move into a full round of Corid. It isn't unusual to have some reddish brown runny poops, as cecal poops are different from normal poops, and happen every 8-10 poops. These also smell pretty bad. They will be followed by normal poops.
Do make sure they're eating, and you can put a meal worm in their feed bowl to encourage them to eat. That action gets them very interested.