Yes they won't starve themselves. I'm sure that different mills feed taste different, just like Mom's recipe tastes different from the same one made by someone else.
They will mostly just scratch around in the hay looking for any goodies that may have come along with the hay. You could give them alfalfa hay. They will eat the leaves in it and not the stems but it will add to their diet and still give them something to pick through.
Yes cut the treats...