I know there's a ghoul settlement, I recall Deacon saying he used to be a farmer and some companions have unfriendly banter when switching. By assigning certain companions to different settlements, can I effectively raise the settlement's "happiness" percentage? Do companions and their settlements matter at all?

The Ghoul Settlement is called the Slog and it is in the Northeast area of the map. Happiness has been bugged since the game came out. The next patch is supposed to fix the happiness bug, but we will have to wait and see. When you need to put your entire settlement in Clinics or use mods to get to 100% happiness, then there is something wrong. I don't recall being able to assign Companions to any Settlement at all and they just take up space. Easy way to find your companions if they are all in one spot.