I personally would just only feed chick feed with no treats until you get to some natural soil. Preferably from where they will be kept at. Most potting soil has stuff added to it. I've never used it so wouldn't know if it's bad or not though. I just like natural
I personally add a dirt/ construction sand mix after a few days of being born. So basically after I see them eating and drinking well. But that's just what I do not sure if it's 💯 percent right. But I've done it for years with lots of chicks.