RE: Fin Size Isonic 125
It should work very well, and since there a two distinct "types" of Hypersonic fins, you could use the different "tip outline" to advantage.
Need a tiny bit more power and early planing performance from the 44 cm Isonic fin..... use the older style (squared off tip) HS 44 cm.
Need a little less powerful fin and want to go faster, try the 44 cm HS fin with the SR6B tip profile. It's got a little less surface area, and a less "powerful" foil, so it's harder to get going, but faster and easier to control once you reach top speed.
There are lots of good fins out there, you just need to begin to understand how each different profile works for you (and against you as Ian suggests).
Hope this helps,