What you're actually trying to say is "cosmetic surgeon," which is like any tom, dick or harry doctor who can do some surgery. They'll inevitably try to sell you on chin and cheek implants.
True "Plastic Surgeons" are in fact craniofacial surgeons trained in cutting all sorts of bone. They can do max fac surgery as they usually do it on little babies with deformities, but they don't normally practice it. Some go into cosmetic surgery as well --for example Stephen Shchendel who is both certified max fac and ps (not recommended by the way).
Yes, different people can give you an idea about aesthetically what you need. I think some people can get away with just a genioplasty to elongate and advance the chin. And yes there is a margin for individual aesthetic tastes. For example, I find Arnett/Gunson too conservative in the amount they advance the chin. They provide great functional profiles which are yes reasonably aesthetic, but I prefer stronger chins on both men and women --and I found Arnett's assessment of me to be too conservative. That said, he was totally open to advancing the chin further to my tastes. A/G are great surgeons and I would certainly trust them. They just don't perform some of the other stuff I want and I'd rather get it all done at one time.