Jumping from 8k to 12k is perfectly fine and is what I do with my naniwa stones. I then follow up on a jnat with a grit of around 15k.
I do not have any experience with the gokumyo stone, but I would think that it is too high a jump from the 8k.

As far as CrOx, it works great on Balsa. I like to use Puma paste. It is in stick form and easy to apply, If you spray CrOx mixed in a water solution it will warp your balsa.