I think the DLC governments and races can appear as AI empires. It's just that the player can't play as them, they don't have their full mechanics unlocked, and they lack all the events associated with the DLCs.
By the way, if you only have the base game, I suggest you pirate it for the full DLCs. You can still subscribe to mods in the Steam Workshop through your legit copy, then play with those mods in the pirated full copy.