You can do either; grower feed with oyster shell, or switch to layers feed.
As Kiki mentions above allowing the hens to accumulate a store of calcium before they start laying can help to reduce some of those early egg abnormalities.
Some pullets take a while to work out their calcium intake and layers feed, assuming they'll eat it, takes care of this without the complication of providing calcium seperately.
My preference in the long term is an all flock type feed and calcium supplement available for the laying hens daily.
It's making the judgement when to make the change over to a higher calcium feeding option that is important. Only you can do this by observing the maturity of your hens.
If you have a male then his interest in mating with the hens can help guide you.