What you want is really easy to do.
Create a new outfit -
Take an existing outfit from Skyrim.esm, right-click and "copy as new record," pick your patch (or make a new one), then name it whatever you want (NOTE: Easier to find your outfits if you start the names the same. I do mine like "AAAA - Rogue Armor".)
Open up the new/existing patch, right-click and "add master" - pick the mod that has the new armor/clothing you want in the outfit.
Select the new outfit. In the right pane, click on the item you want to change, and hit F2. Find the item you want to replace it, then hit enter or click out of list. Add new records for additional items, or remove items you don't want.
Add in NPCs you want to wear the new outfits -
Find the NPC you want to wear a new outfit in Skyrim.esm (or other mod like Bijin, etc.) Right-click on him/her and select "copy as overwrite record" and pick the same mod you put the new outfit.
Navigate to the mod with the outfit, and select the NPC. Scroll down to the "Default Outfit" record, hit F2 in the right pane, and then pick your new outfit (easy to find if you name them all similar.) - NOTE: if the NPC doesn't have a default outfit, you will have to right-click the record and select "add." You will now get a Null record field that you can hit F2 and select your outfit.
Voila, all done. Doesn't change the NPC in any way other than the outfit.
BONUS: If you know how LVLIs work, you can build your new outfits to pick from different lists. That way, your NPC's outfit will be a randomly selected option from your items in the LVLI ... and if you don't like what they are wearing, you can open up the console, select the NPC, and type "resetinventory." all done, new random outfit.