shorter fin = easier to turn in non planing winds. My upwind 360's and heli tacks improved when I got a really small fin. Easier to push with backfoot and get the board rotating into the wind and through the wind.
I think with a small sail you will still move. Depends on what size board though. I typically learn new freestyle on a 5.0 or 5.8 with a 99L board. I only weigh 150 pounds so it isn't too challenging or sinky (I can uphaul etc).
Hope this helps...