I'd like to add in, if you think the snake will take Frozen/Thawed, you can always try it. There's no rule that you must use live prey.

Agreed.. breaking the ribs? Never even heard that one. Snakes are remarkably flexible, and I doubt highly that a large prey item would ever manage that.

If the mouse is about as big as the thickest part of the snake, it should be fine. Like the other poster mentioned, you could also try rat pups of a correct size and it would not be able to harm the snake.