I wouldn't add any straps until you've sailed the board. It light winds you don't need to be in the rear strap, in fact on the F158 one could sail very well in light winds by having the rear foot on the front screw of the rear strap.
If it is for shallow waters, I don't think you will gain that much by using a smaller fin. I cannot imagine doing 15-20 knots and only having 10cm of water underneath the fin. With Formulas you can easily travel long distances and go both upwind and downwinde well, so you're better of walking and sailing further out.