I wounder if there is any stability method to prove stability of control system consist of linear control (i.e., pid ) plus nonlinear control (i.e., SMC) in parallel?
In general, most stability analysis that apply to nonlinear systems and their control apply to linear systems as well. The reverse is not always true.
So something like Lyapunov Stability Criterion or Popov Stability Criterion, both of which essentially apply to nonlinear systems, should be applicable to the hybrid (linear + nonlinear) systems that you have in mind.
Selection of the stability functions may become more involved, but the techniques themselves should work !
Indeed it would be more rigorous this agree on the word hybrid (linear + nonlinear) as was noted Prof. Sanjoy or switched and hybrid so in this case we will use Multiple Lyapunov Function. I propose a rather interesting paper attached.