Thanks for your input, guys, I really appreciate it.
I decided to keep the 38cm fin (for my 7.5) and ask the dealer for the 32cm fin I'll use with my 6.0.
I don't think I'll be getting another smaller sail soon (I survived last season on 7.5 no-cam, and some days were just nuclear, people sailing 4-5-5.0 and me 7.5, hehe, so with my new 3cam 7.5 and 3cam 6.0 it should be even better).
Now I'm just wondering - how will I decide whether to use my F-type 148 with 7.5 sail or my new Carve 111 with 7.5...
Also I'm kind of thinking about putting my old longboard aside; the sensation of planning with my FT/10.0 in low wind and flat water is just so great. On the other hand if there are many light-wind days I may be just getting the longboard out for some cruising.