Thanks Sean for your helpful info. I will go for the 9.5m (still undecided about the NP to choose: racing or slalom).
I will buy for sure a R16 for the 9.5 but I'm not completely convinced about the size 50 or 52 cm. My usual race are on a 8 course and in light wind with strong sea current. I've never ridden an isonic so forgive my hesitation.
The moment the wind picks up I'll move to the 107 with a 7.8 (fin SL4 40 or 42?). Just if I find those strange days with a very gusty wind and big lulls I'll take the 127 with the 7.8 and a 46 fin. Just for now (no more money in my pocket after buying 2 isonic, 3 race sail and 2 deboichet fins) and for these few days I'll use my old SL1, but I'll move to R16 as soon as possible.
Your suggestion about the fin size is very welcome.