Ben, I won't get into the fray of which of those boards is best...but if your only concern with the Shred Stixx is loosening up the feel of the tail you can achieve that easily. Jerry uses Future Airs fins in a thruster pattern. So the trailing or middle fin is removable. That will give you a twin fin. Now that will loosen the tail up, making it very easy to break loose. If you STILL want a looser feel, you can size down the side fins. I do believe that Jerry ships his boards with 3.5 or 4 inch side bites (SB). I'm sure Jerry would let you order it with 3.25's -- the same fins on the XBC Grom and most of the Inland Surfer boards.
If you STILL aren't happy, and have a bench grinder you can trace an outline on the fins and grind away...effectively making them as small as you want. I have a few 1" futures
The one thing to remember with smaller or shorter fins is that they reduce drive compared to deeper and MORE fins. You need to find a balance that works for you OR if you principally want to bust airs you'd leave the thruster pattern and 3.5" or 4" SB's alone.