I don't think there are any people with Ditto in the game. Could be wrong though. Hmm..what about that Friends Code thing? I think you can trade with people whom you have the Friends Code of, right? I think there's a community on LJ for that? Or try the Serebii forums.

You can only get Skull fossils in Diamond. To get Shieldon, you need to trade with someone who has Pearl =D

You get Piplup. The eggs always hatch into the 'youngest' form of whatever Pokemon's bred. So you get a Budew if you breed Roserade or Mime Jr if you breed Mr Mime.