It's perfectly fine to use the medicated feed with the Corid in the water- the feed has such a scant amount in it that they won't be overmedicated- there is excellent documentation on how much is too much, and you won't be anywhere near that threshold. It's important to note that Amprolium is not like most medications in that it simply blocks the organism from utilizing Thiamin. This is not at all a major issue for the duration the chicks will be on it (5 to 10 days) at full dose, but it's enough to cause the life cycle of the oocysts to be fully interrupted and the population will dwindle dramatically, giving the chicks a chance to repair their guts and recover from the damage.
The bedding should be changed often, but it sounds like you do that already.
It may help to give them something like BabyCakes from
Tractor Supply (C&S is the brand) for a nutritional boost and probiotic supplementation. I would certainly be giving them that or putting baby parrot formula powder on their feed. Also, live culture yogurt should help a lot, as it will foster the healthy flora in their intestinal tract.
I would avoid Sulmet since you have Corid- I only recommend it as a last resort to those who can't find Corid- it's MUCH harder on their systems as it's a true antibiotic, and a pretty harsh one, at that. It will kill off any remaining good flora, and you'll have a harder time bringing them back to good health. Corid is much better on its own.