I have a bunch of rectangles, different sizes. How can I make them all have rounded corners with the same radius easily? I've tried copying one and stretching, but the rounding is stretched also
By default stretching affects the corner rounding in proportion. However you can de-select the second icon in the "Affect:" part of the "Tool controls" bar, this will allow you to keep the size of corners while stretching objects.
So if you have not created the rectangles yet, you can copy the original one and then stretch it to the desired size without affecting the corners.
Remark: It is advisable that you also de-select the first icon in "Affect:", else the stroke thickness will be affected, which is probably not what you want.