RE: Kombat 107
GEM, I don't know your weight and if you tried the K87, but it actually handles 6.5 (well, 6.3 in my case) very nicely with the right fin and just using widths as a sizing tool really does not tell much when comparing different types of boards (like Carve vs Kombat). My freestyle and Slalom boards have similar widths for example and cover VERY different ranges. In any case its kind of academic as Chris got he 97 (which is a good choice considering the info given).
The fin advice is excellent though and I will add a bit: The 07 Kombats come with a single crossover fin (except for the very first which still came with the two fins from 06). Its a Curtis design, kind of semi rake and sweep definitely more powerful than the Natural wave fins but a lot turnier than the 2006 freeride flow. I would say the 28 will handle a 6.5 on the K97 nicely, especially at 65 kilos. You will loose a bit in pure straight line blasting compared to the (bigger) freeride flow but the turning will be far better and this is why the new Crossover fin fits the character of the board better and complement the board nicely both with the straps set inboard and outboard. For smaller sails you might want to complement with a smaller (wave) fin. I love the Drake Natural wave myself (super flexy) and I think their almost a revolution in fin design when it comes to the range of sails each fin size can carry. For B&J:ish sailing you can safely go up a bit in size and I would say a 26cm Drake Natural handles 5.0-6.5, a 25cm handles 4.5-6.0, a 24, 4.0-5.5. So, a 25cm Drake Natural will for example be a pretty good fit for your sails while still offering some "reserve" for a smaller sail or maxed out conditions on the 5.25 (for proper waves with the 5.25, the 24 cm will be the best fit).
For the Carve, I suppose GEMs fin advice is spot on and the most important point he makes is that you will get A LOT more from both your boards if you get a few extra fins.