Forbidden Broadway: The Next Generation is review-proof.  Creator/Writer/Director Gerard Alessandrini started parodying The Great White Way in 1982, and The Triad Theatre (158 West 72nd Street) was packed with show-tune types for its first reincarnation in five years. Running through November 30, the 80-minute revue is the best kind of tried-and-true evening, matinee or celebratory time out with a great cast and Fred Barton at the piano. Alessandrini didn't have to look hard for new material.  Most of the skewering is for movies turned into musicals - a formula with mixed creative and box office results.  Moulin Rouge!, Tootsie and Beetlejuice  all get