15 are standard on the Q76 and they are not only smaller but also a tad softer. A 13cm fin is now also available (might be good to push your distributor to order some as these fins were not ready to order at the time when the first orders for Quad boards were placed). I don't think the 11cm has materialized yet.
Personally (at 70kg) I've been happy both with 16+11, 15+12 and 15+11 which I have been running in the 66 to 81 liter Quads. Only in some super high wind (mean wind up to 47 knots) I felt more secure on my twin than on my Q66 with 15+11, maybe indicating a smaller setup would have been nice. In any normal conditions I've encountered the 15+11 have for sure been small enough. I think it will be more an issued about trimming the character of the board (de-power the turn a bit at the expense of maybe not as amazing upwind). But I haven't used the 13cm yet so I will be able to say more within a few weeks.