This is a tough question since I personally think the 70 is rather easy to pin down. I'm actually not even sure a smaller fin would give less bouncing (but it does give smoother wave riding).
Have you tried the 23 on the 70? If so how do you feel the board changes when you go from 23 to 22 (with say a powered up 5.0)?
Maybe you will feel a smaller fin helps. We are talking rather subtle things here and stance and style can make different people experience differernt things. I think the 21 is allready a fair bit smaller feeling. I'm rarely on the 20, but when I am its sweet. So, take the 21 for a safe step down and the 20 to _really_ feel how a small fin will change the board.
I've never tried small stiff fins on the EVO so I can't say- A good compromise might be the 08 Drake natural which is a tad stiffer. Maybe you can borrow a stiffer fin
in from somebody to test before you buy?