When you download a DLC, does the campaign creation screen allow you to choose whether to play the original version or with the DLC added? If it doesn't, can I disable DLC's somewhere else? And how would I go around doing this?
You can disable DLC at the launcher. Simply click the DLC entry on the list on the DLC tab to add or remove the check-mark (Same goes for mods on the mods tab).
If you are using the steam version you can also disable DLC that way, Simply right click on the game in steam and go to properties -> DLC (tab). Uncheck the checkbox for the DLC you want to disable.