I've used the Design Lessacher 32 and 26 in the Serenity quite a bit, as well as using Tangent Dynamics Reaper 36 and 39 cm.
Also the True Ames Shallow Water weed designs work well.
I think what you will find on the Serenity is that you don't need nearly as much fin as the huge wide blade fin supplied with the board.
Also, you may find, as I did, that you don't need sails larger than 7.5-8.5.
You need to consider the additional weight of a larger rig against the performance in very light winds.
Hope this helps