1. The Gambinos in their prime were said to be the largest, with over 500 made men.
2. The guy you're thinking about was Carlo Gambino's nephew. He was kidnapped for ransom and murdered anyway by James McBratney, who in turn was killed by Gotti and two others. Gambino hired Roy Cohn to defend Gotti, who got off with a manslaughter convinction and served less than five years. Gambino's sons ran the Garment Center trucking rackets for Castellano and Gotti.
3. The Bonanno boss who was gunned down was Carmine Galante.
4. No one really knows exactly what happened at Columbus Circle in 1971. The first theory--that Crazy Joe Gallo arranged it--has been pretty well debunked over time. The most logical current scenario is that Carlo Gambino ordered it to shut Columbo's mouth. One little factoid sticks in my mind: no one knows the background or motivation of the gunman, because he was instantly killed by Columbo's bodyguards. A bit convenient, don't you think? An inside job?
5. I'm sure some cops and federal agents were killed by mobsters over the years. I'm guessing that they were crooked, and there was a "conspiracy of silence" on both sides: neither the cops nor the mob wanted to acknowledge that there were such things as crooked cops or mob murders of cops.