Effect of the zygomaticus muscles, major and minor, injected with Botox? Is there a safe amount without affecting the smile?

I am not sure what your treatment provider is trying to achieve by injecting the zygomaticus major and minor muscles. Any weakening of these will cause a droop in the corner of the mouth.  These can be treated in isolated cases for major asymmetries, but to the best of my knowledge, it is best to avoid injecting them.