I have a 686-6 that I was thinking of getting plated. I have a couple of questions, can you nickel plate over stainless, and if so, who would you recommend to do it.

Why? If it's shine you want, just polish it with Mother's.
If it's a super-durable finish you're after, go with Robar or one of the other hard-chrome type finishes. There are several; two places I'm familiar with are George Roghaar and the aforementioned Robar, or a quick google of "hard chrome finish" should yield more.