You can trade Pokemon between any two Generation IV games, as they are all compatible. To trade between these games, you only need to meet two requirements:
In Diamond, Pearl, and Platinum, you need to have obtained the pokedex from Professor Rowan. In Heartgold and Soulsilver, you need to have obtained the pokedex from Professor Oak.
In all Generation IV games, you need to have at least two Pokemon in your party.
It's as simple as that. You've already obtained the pokedex for Diamond, Pearl, and Platinum, so as long as you have at least two Pokemon in each of these games, you should be able to trade between them. The same is true for HG/SS.