setting their outfit in the npc > inventory tab forces them to wear that outfit, thats what i used for my pirate mod, i made a copy of some raider boss (forgot the name) and then changed his clothes and looks based off of that, he always spawns in the outfit i selected search for outfit, make a copy of one, rename the ID and delete the other clothing items from it, keep that box open, go to the armor section then drag-drop those items into that outfit box, save it after you make your outfit and assign it to the npc of your choice, theres no chance theyll spawn in anything differently or anything standard, just make sure you edit the correct npc or make your own and place them in the world as for the BoS stuff i cant say that i know what that keyword means, but i know that changing their outfit works, in my work in progress part of my mod i have 3 npcs that i have set outfits to and they never wear anything differently