RE: too long fin
Probably you may smoke epoxy dust when sanding your fin after shocking it in shallow water. On slalom/freeride boards, I use fins up to ~2 cm above the OFO without too many control issues. This is valid for the lower wind range. When overpowered, the board starts wheeling and I have to go down in size. However, this is not only a matter of length : I remember a 31 cm carbon fin very thick and stiff which was almost unmanageable on every board.