What I find strange is how the whole dwarven Ambassadoria thing works now the Deep Roads are overrun by darkspawn. The dwarves apparently live under Minrathous, never stepping under open sky, but I very much doubt that the Magisters would appreciate having tunnels open to the Deep Roads underneath their capital city, so how do the dwarves keep in contact with Orzammar except by utilising their surface dwelling kin? It seems odd to me that these subterranean dwarves in the three main cities should have such status when the lyrium would never get to them without the surface trade.
As for dwarven slaves, well back in Legacy, Corypheus mentioned about slaves of the dwarves, so apparently they keep slaves. Since they are a somewhat closed community, presumably the castless are probably kept as slaves by the dwarves under the Tevinter cities in addition to other races that they purchase from the surface.