I quickly watched the video and didn't see what you saw. Can you give me a link + a time where you can see this big fin back quad setup? I did see some of them used a convertible with a rather big fin in the back (ie set up as a tri fin). The Quad convertible goes well like this. It's not what I prefer myself, but it's good that such a setup also works cause it opens up even more tuning possibilities.
mikel: Yeah, wave fins from 21-25 is pretty much the right range to use it as a single fin. As a tri fin, you should try smaller in the middle, particulalry if you keep the big 16cm side fins.