Yeah, the DLC is a side-scenario separate from the main game, accessed via a new option in the title menu. There shouldn't be any spoilers for the main campaign,as long as you've played the first four or so chapters . As you go through the DLC you gradually unlock the content in it for the main campaign. Completing the DLC has no effect on the main game other than allowing you to recruit the new characters and access some new features.
It was more challenging overall compared to the main campaign on the same difficulty. I had trouble on two stages in particular while playing on hard, but only had to restart on one of them.