First choice is to put the shells on the side and let which ever pullet pick at the shells, if their bodies are calling for calcium. Dead easy solution.
The other is this. The sex links may begin lay at 17 weeks. Not the least uncommon, while traditional breeds don't often start before 22 weeks.
Make the feed change to layer for all of them at 20 weeks and call it a day. This is no where near as mission critical as it is often made out to be. Yes, the GSL would get their calcium boost a week or two late and the Doms a week or two early. No big deal. ALL commercial feed has calcium, it's just a matter of how much. Trust me, the birds would be just super, duper fine whichever way you choose.