1) In the field, black iron pipe commonly refers to carbon steel pipe such as A53, A106, A120 etc. without a galvanized, painted, or tape coating.
2) Typically it is socket welded sch 80 for 2 inch and smaller. Hook ups to the generator should be an approved flexible hose, as opposed to hard piped.
3) Buried fuel piping is typically installed in double containment piping. The project I did used 1 inch carbon steel piping contained inside a 3 inch fiberglass containment piping system.