I'm trying to figure out how to properly mod DLCs before I start just poking around and possibly mess things up. Right now I have a mod that is mostly setting and perk changes, though I'm planning on item changes, too. In order to do the same in the same mod, would I just choose the DLC I want to mod in the GECK as the master file and then my mod as the active file? And, if so and someone were to play this mod and not have the DLCs, would that cause problems for them? Thanks.

If you make a DLC required, the mod will either crash the game or not load at all if the other people do not have the same DLC.
Thus, the term 'required'.

I don't want to make it required, I just was hoping to set it up that if someone has a DLC, the mod will work for the various items and whatnot specific to that DLC. For example, I'm removing all zoom effects unless the player is using binoculars or a scoped weapon. Obviously, I want this to apply to the police pistol and .45, along with all other DLC weapons. However, if the player doesn't have the DLCs, then it shouldn't affect him/her.

What I would suggest then is to make two versions of the file. One with the DLC stuff, and one without. Then the player can decide which to install based on what DLC they have.