Is there a mod out there that supports the voice types of NPC's, so that the player can make them a follower if the relationship rank is high enough? It's odd how Mjoll can be a follower with a nord voice type, but when it comes to custom NPC's the option isn't open

It isn't just voicetypes and relationship rank that govern who can be a follower--it also has to do with whether they're in the PotentialFollowerFaction.