sailed the Isonic 111 some weeks ago. Quite wide sailrange can be carried by this board. Had a 9,0 on. Would suggest 42 or 44 for 8,4, for the smaller sails from 6,5 - 7,5 36 to 40.
I use myself MFC. They are quite nice,.
But for sure the Deboichet fins are hard to beat, no discussions. Vector should work fine, too.
Good luck for your races,