RE: S-type 115
I use my S-type 115 from 4.7 to 7.5. This is max range and it works pretty well no matter how I tune the board (straps in/out etc.)
I'm 97 kgs.
For 7.5 and 6.5 I use a 37 cm freeride fin. It may not be as loose as the smaller standard fin but early planing and bottom end is way better.
For 5.75 and 4.7 I use the standard 32 fin. It works pretty well in this sizes. A back swept 28 fin could be fine for the smaller sails depending on your style.