What happened to the 31 vm Drake Shallow Water fin that was supplied with the board?
Larger question (to my way of thinking) is why is a 90 KG (198.4 lbs.) sailor sailing a
175 liter Rio S (Small 175 liter... 269 cm long...76 cm wide)?
At 90 Kg. I would have suggested at a minimum the Rio M (Medium 195 liter... 275 cm long.. 80.5 cm
Ok, now the tough question.... do you have weeds or shallow sandbars/obstructions where you are sailing that require a weed fin?
If you have no weeds, you probably would need a 52-58 cm vertical fin to get the Rio S to plane pretty
early (maybe 14 knots for 90 Kilos on an 8.0 m2 rig).
On your 2nd question, the std. Tuttle fin will fit nicely in the deep Tuttle fin box, but you will need fin screws that are 20-30 cm longer to reach down to the brass cross nuts (barrel nuts) in the head of the fin.
Hope this helps,