How would I change an NPC's hair, using one from another mod? How would I reference the other .esp file?

The easiest way to do it is to copy/paste the meshes/textures of the hair you want to use into the appropiate folder in the regular game folders. You may need to also copy the folder structure to match if it doesn't already (i.e. Data/textures/characters/hair and Data/meshes/character/hair). From there you would have to set up the hair in the geck character/hair pulldown menu (give it name, etc.), and then add it to the npc.