It doesn’t get very cold here and leaving (misplaced) eggs overnight with -5°C never caused cracked eggs. I can imagine harsher temps are a problem. In combination with rather soft shells they probably crack sooner.
I have two kinds of feed available for my mixed group. Chick crumble and layer pellets. I can’t buy organic feed for a mixed flock for a reasonable price in the Netherlands. And this seems to work just fine.
The oldies that haven’t laid an egg since last summer /autumn mainly eat the chicks crumble. My oldest prefers the wet chick crumble I make once a day.

The neighbour cats, who come to dinner uninvited, seem to prefer the wet chick crumble too.
I always have crushed oyster shells on the side and give crushed egg shells back. The young hens that lay, eat the egg shells as a treat.