This gives you a clearer view for you:
Let me answer you questions.
- Kernel scale is just for RBF kernel? Yes, you're right. This is a hyper parameter.
- use 'KernelScale' to apply a kernel scale of your choice. The option 'auto' evokes the heuristic method.
- according to the documentation, the default value for the kernel scale is 1.
It does not elaborate on the heuristic method but I bet it will use a method like cross validation.
I would optimize the parameter via hyper parameter tuning, e.g., bayes optimization.