No there two different games and it would make Pokemon diamond much easier to complete. Besides on mystery dungeon the Pokemon don't get ev so they wouldn't be able to reach their top stats which they would on Pokemon diamond.
yes you can just trade like you normally do
Nothing happens you just trade him and he leaves and you go on with your Pokemon journey
No, you most likely have to trade from Pokemon pearl, in my knowledge, at least
Yes
you can get it by getting a pichichu in pearl trade it to heartgold
No, but you can trade Pokemon only in HeartGold to Black
you can trade pokemon from heartgold to black but you can only transfer the pokemon in heartgold not trade it from black
yes. you can trade Pokemon from diamond, pearl, or platinum to heartgold or soulsilver.
trade pokemon
no, but use can use poketransfer from heartgold to white
you can trade Pokemon from heartgold and soulsilver to your diamond,pearl, or platinum.
You need to have a Pokemon hold the item then trade the Pokemon.
Trade it or have it as the starter pokemon.
Trade
Trade!
You get him in trade.
Trade