Adblock Plus blocks all requests as required and should do significantly better than either Flashblock or Ghostery. Adblock Plus can block Flash easily and even individual requests sent out by Flash without blocking the entire object.
1) Flashblock cannot be considered a security add-on. I looked into it just recently, Flashblock runs in the context of the webpage and can be easily manipulated by it. For example, a website can disable Flashblock in a trivial way.
2) From all I know, Ghostery cannot block anything - it will only report privacy issues (including web bugs that have been blocked by Adblock Plus). EasyPrivacy should already block anything that Ghostery will notify you about and in a more reliable way (last I checked manipulating Ghostery was easy as well).