Migrate, trade, or catch them all from all versions of the game.
you have to get to dialga which is a ledgendery Pokemon or, you might be able to trade palka from Pokemon pearl to Pokemon diamond.
Yes, much like it's predecesores (Ex. Pokemon Gold and Silver could trade to Crystal) Pokemon Platinum will be able to trade Pokemon from Pearl and Diamond.
you are not able to get togapi in Pokemon diamond and pearl
you can't but with action replay you might be able to
Of course you can. I played my Pokemon Diamond with my cousin that has Pearl.
Rock Climb Any Pokemon Can Learn It If It Says "ABLE"
Yes.
You can't, because Pokemon black was made after Pokemon diamond, Pokemon diamond wouldn't be able to recognise any of the new Pokemon. So to solve this problem they stopped all trading from black to any other game apart from white. You can however get your Pokemon from diamond to black by using the poke transfer, yet you need to have won the league for the first time to be able to do this.
turtwig, piplup, chimchar
You have to wait until you beat Pokemon Black/White and obtain the National Dex before you are able to transfer Pokemon from Diamond and Pearl.
Beat the Pokemon League, and you will be able to find all but one of the fossils of the ancient Pokemon. (For Diamond, all but Shieldon's fossil are available)
You are not able to catch the regis in Diamond. You can catch them in the Game Boy Advance version of Pokemon Emerald.