RE: Carve 131 (2005) fin
You can use a smaller fin, but your upwind and early planing performance will suffer.
Why do you want to use a smaller fin?
Sandbars and water depth problems?
You might go a little faster with a smaller fin, but getting the board to plane is going to be much more difficult.
If water depth is the issue, have you considered a weed fin?
Hope this helps,