Because the sizes have to be the same and be compatible with each other.
That would depend on the brand of the toilet as some screw on others pop into place.
Mine has neither and it works perfectly I install this type all the time
Generally no, depending on how old the toilet is (because older toilets came in all shapes and sizes, but newer ones are more standardized). If a tennis ball plugs the waterway -- and it should -- no...
This would depend on what brand of toilet paper it was.