Don't feed them medicated starter. It won't help peachicks. You need to get some Toltrazuril in case of coccidiosis. You should be able to get some from your local co op. There are a few brands available here but
this is one example. All of the ones you can buy are for cattle but they will be fine. I can help you with the dosage amounts if you need it at the time. There is a poultry version but you need to get it from the vet and it will cost you a whole lot more.
The best thing you can do is keep them regularly wormed with Fenbendazole and observe them vigilantly. I know KSKingBee treats his with Toltrazuril monthly as well as worming but that's with large amounts of birds. On a small scale, if there's no build up in your land, you can proabably manage with just keeping the Toltrazuril on hand and only using it if you need to rather than doing it every month. You definitely need to have it on hand though. Don't wait for them to be sick before trying to order it.
I fed mine
this turkey / game bird starter from Red Mills. I was able to get it ordered in by my local co op. They order in the turkey / game bird grower regularly for me for my adults. There is also
this from Kiernan Milling but it's hard to get unless you buy bulk quantities. Both brands are only available seasonally so you could check with your co op when they can get it.
The one I used for the peachicks was 30% protein but mine were out and about with the peahens. If they are in a brooder they can develop leg problems if they have too much protein and not enough exercise.
@KsKingBee I'm sure you know what amount of protein is ok for peachicks raised in a brooder?