May I ask why you want to use a NP Hellcat on an Isonic ?
I'm a big fan of NP, but not of the Hellcat
If you don't want to go for the expensive NP slalom/racing sails, you still have the NP Heliums as a good alternative. These sails are MUCH better and faster than the Hellcat.
Are you afraid of the cambers regarding jibing ?
Or is it because you don't have the correct NP mast ?
I can tell you that there are a lot of good and cheaper alternatives for the expensive X6 & X9 NP masts.