Adding new merchants and creatures/npcs

After working on some items, I've started to consider releasing my item pack and then some, but I never delved into adding NPCs. Years ago when I modded before, I was thinking about adding a merchant to Waukeen's Promenade, but I never did. Does the whole area file have to be edited? I'd like to do what I can to keep my mod from being incompatible with others.