In Formula Experience, you have two solution for the fin the R13 rake + 6cm and the R19 rake + 8cm. Their only one stifness available and you can not use other fins than the Drake one.
The F161 and F162 are deliver with the Drake R19 rake + 8cm (same fin as the FE160).
In Formula Windsurfing is different, you can use customs fins. The R19, R20 & R17 works well on this 2 boards.
All the best