The crossover 24 cm fin of the K79 feels a bit small with a 6.2 sail when the footstraps are set on the freeride mode (outboard and back). Something in the 26-28 cm range would be far better IMHO. However, when footstraps are set on the wave mode (inboard and front), it's OK, althought a pure wave fin would shine better for true waveriding (softer & more twist than the crossover fin).
Pretty much what I guessed, that the crossover 24 would kind of work for wave application with a 6.0 (6.2). I've used my K87 with a 6.3 and the Natural 25 and 26. Thats probably the ideal setup for waves, but there's always a question if the upgrade in turning performance relative the crossover 24 is worth the money. In any case, I think you will like the xo24 for onshore kind of jump and backside riding oriented sailing. The more freerideish you 6.0 B&J sailing is, the more sense a 26 crossover will make (or even 28 for pure freeride sailing with straps out). But since you will get the xo24 with the board, you can simply try it out and know you have several choices if you don't like it. A natural 23 is pretty much "a must" though since it will expand the range of both boards so much.