Are all rear monster shocks compatible? Or are there different sizes?
They're compatible within frame families. One family is the original 851/888 based frame and the other is the ST based frame. The new ST frame is used on the S4, S4R and '02+ 620, 750, 800 and 900.
I can't comment on spring rates though. Since all the 2V bikes weigh close enough to the same, they may all have the same springs. But if Ducati was targeting the smaller bikes at presumably lighter riders, they may have softened the springs on those.
Of course springs are easy enough to swap.