You can feed any bird = non medicated chick starter, grower, or flock raiser. When you have a mixed flock, it's just easier. Offer shells on the side, and you're good to go. Young birds seldom mess with the shells, as their bodies aren't desiring the high calcium.
The problem with rushing the layer to young birds is that many simply don't come into point of lay until 26-28 weeks of age. Feeding super high calcium layer formula just isn't indicated for non laying pullets. While there'd be little you could detect on the outside, internally, there is just this risk of renal/kidney damage from the calcium buildup. It is possible your pullets won't lay for another two months, far too long to be on high calcium layer formula they do not yet need,
Once a pullet goes into lay, she can use the calcium.