Similarly in skirmish and multiplayer you can play as any race you own. Any races you don't own the DLC for will still appear on Mortal Realms, but you won't be able to play as them. They can all be used in the Mortal Realms Grand Campaign map (enbled when you own both games).
All of the factions from TW Warhammer 1 can be enabled in Warhammer 2 if you own both games on the same steam account (and own the relevant DLC for DLC races from Warhammer 1).