Yes, amprolium is the active ingredient in Corid. I would keep using that with the chicken chicks. As KSKingBee said, make sure you don't give them vitamin B or any multivitamin that contains it while using amprolium.
It's good that you have the Toltrazuril ordered. I would only use that on a chicken in a dire emergency because the safety in eggs and meat is unclear. The amprolium should be sufficient for them anyway if you follow the dosage instructions correctly and catch things soon enough. I would do one round with them now since they were with the peachick. After that, I don't recommend doing preventative treatments with chicken chicks because it shouldn't be necessary and you want them to build immunity. Just keep an eye out.
If you do hatch more peachicks, treat them with the Toltrazuril monthly until their first winter. You don't need to do it as soon as they hatch. I don't remember when exactly to start but I think it might be about 2 weeks old is that right 
@KsKingBee? Adults don't need any preventative treatment because it would be very rare for them to be affected unless their immune system is compromised by something else.
Do you know which parasite your vet gave you treatment for and what the name of the treatment was? I'd be interested to hear that.
I'm so sorry about your poor little peachick. You did your best for it and it's great that you're wanting to be well prepared for the future. 
This is a great post about what's good to have on hand.