A diet containing too much calcium can be a serious problem for young chicks and growing birds under 18 weeks of age. The recommended level for calcium is 1% or below for these birds; layer feed has up to four times this amount, so you should never feed it to birds under 18 weeks.
I dont recommend starting calcium until they start laying or start layer feed.
And a whole eggshell is about 2000 grams of calcium compared to a gram of oyster shell which is 1000 grams. So go ahead with the egg shells. Some people dont use oyster shell but use egg shells, grubb terra black soldier fly larvae (75% calcium) and leafy greens which are all enough for calcium and hard egg shells.