I wouldn't give them the extra calcium until the start laying because if they eat access calcium when they are young it can cause kidney problems and spinal deformities. Those egg shells are probably okay, but since by the time you actually start feeding them to your pullets, you will have their...
Welcome to Backyard Chickens! :welcome You can give them discarded egg shells as long as they don't resemble egg shells whatsoever or they will start egg eating habits. I agree with the post above. They can't replace oyster shell.