The obvious answer (to me anyway) would be to open up the fin box on your iSonic, and continue to use your fins that fit all your other boards.
I just received a 2010 iSonic 121 WC and I noticed immediately that the fin box is not the std. Cobra box
we have seen for the last 10 years.
Obviously they have redone the interior mold for the fin boxes on the new iSonics, and my guess would be that they spec'd. the original Larry Tuttle design dimensions.
So, we may see better (closer to Larry's original desin tolerances) quality control on all Starboard fin boxes, but then again maybe only on the "hand crafted" high end, very expensive iSonics.
Enlarging the interior of a fin box, already installed in a board, is a lengthy hand work process unless you have a fairly large milling machine and some very special very long end mills and a way to swivel the
cutter to the front and rear taper angles.
So, working the fin heads down seems the logical way to fix this issue.
Maybe a better way to go would be to make up side shims for your other boards, and then epoxy the
shims into the fin boxes to decrease the interior width.
This is something you could do in a pretty straight forward manner, without any large precision tools required.
You could cut off the head on a broken fin, and carefully tap a couple of holes and grind/file/sand it to the correct width/length/depth, and use it as a "master" to "cast" the shims into the fin boxes on your other boards, then work down all your fin heads to this spec. The tapped holes could be used to "pull" the master plug out of the fin box after the epoxy slurry you use to hold the shims in place has cured.
Hope this helps,