Leaving an egg or two under them won't break them, it will encourage them to set. Which may make them more peaceful as they are setting and not lumbering around screaming.
I would definitely leave a clutch for your hen to set upon if you want to foster feed store chicks. She needs to be in a good brood, for at least a couple of weeks, before you attempt to place chicks.
Fostering can be a bit tricky, so have a plan B if the hen refuses the chicks or the chicks are clueless to stay with the hen (the more likely of the two).
LofMc
Thank you for the advice, and yes. But the 4 times we have fostered chicks the mom has done just fine
