Warhammer Dawn of War Series Exp?

I am thinking of buy the warhammer dawn of war game and i have a few questions about the expansions. The latest expansion Soul Storm is (stand alone) meaning you dont need the previous games in the series to play correct? Well would you benefit from having them all though? Like Soul Storm doesn't include the units from the winter assault expansion? Or you only get the maps that come with it? None from the other expansions? And how would the online play work exactly? Can i play with someone who just has Dawn of War? Or someone who has the Dark Crusade (stand alone expansion) Would I need everyone in game in the series to get the full experience? I really don't care about the campaign modes though. Thank you.Warhammer Dawn of War Series Exp?
The standalone has all the previous factions from the past in campaign and skirmish however you can not use the previous factions in multiplayer.

With only soulstorm you can use the sisters of battle and the dark eldar