Should same sized peltier elements be cascaded?
For making a fridge, do not attempt to stack your elements. What happens is that the lower element has to deal with the waste heat from the upper element.

Stacked Peltiers are used when the final temperature must be lower than a single element can provide, but the amount of thermal energy it can remove is much lower than what a single element can handle. That is why the top element has to be smaller than the lower ones.