Old pewter has lead in it, and I would not use it on a mouthpiece. New pewter does not have lead and can be okay to use. I don't know if there are any esthetic rules regarding the mixing of metals. Surely someone else will know. Instead of pewter, why not use Cerrosafe, or cerrobend? They cast at a low temperature( less than 160 F.) and give a shiny finish that does not tarnish like old pewter does.