Hmmmm.... the Serenity fin always seemed way too big for the Serenity (in my opinion and how I sailed the Serenity) but it seemed similar in stiffness to the Apollo fin which was OK on the Apollo (also seemed a bit big, but if you put a 10.0 m2 + rig on the Apollo you might need a fin that big) but I found that a regular Deboichet Concept 68 was more to my liking with a 9.1 m2 race rig on the Apollo.
I never needed the big fins on the Serenity as the biggest rig I normally used was a 7.5 m2 Severne Glide or Sailworks Retro.
Also, Jay has/had an F-Type 158..... quite a different animal than a full on Formula board.
I found the F-Types worked best for we with smaller fins.
Hope this helps,