Pat, In my area, black iron was for many years not to be mixed with galvanized for gas service. Recently, this was reversed, and supposedly there's no problem using galvanized. The primary reasoning is combination of dissimilar metals, just like the problems created by mixing galvanized & copper water lines.
Personally, I'm not totally convinced, so I stick with all black iron (how many times have we been told something's okay, only to have this reversed later with a statement of "Oops!"), but I do see it done all the time in where it's been mixed for years with seemingly no problem.