So glad to have you with us. I'd offer the oyster shell in a separate container, usually they are pretty good about eating it free choice and only eat it as needed. But you can also keep an eye on the younger ones and make sure they don't go crazy with eating it! Good luck to you & your family!
I think its ok unless you are already giving them a layer feed ( like layena for example). Laying feed blends have ample amounts of calcium and if you give a chicken too much it can give them kidney failure. If you are feeding them chick starter or a finisher or a raiser, then you should be ok. Soft shelled eggs mean they aren't getting enough calcium or just too many treats.