Don't worry, you won't be planing unless you find some 50+ wind, and even then only briefly. I like a medium-sized dolphin for sup sailing, something in the 7-8" range. I've tried reversed thrusters, and they work OK. I might be reading your post wrong. are you thinking about a fin as a centerboard? Some of the early SUPs, most notably the Starboard 12'2" and the 12'6" had tuttle boxes to permit installing a centerboard fin. starboard even made a flavor of 12'6" with a swing fin that could be kicked into place. But no, you don't need one unless you're sailing in very light wind, like sub-10 MPH in the sit up and beg position.