The NPC you transfer must have a Melee or Ranged weapon skill of 40 or better. Also the NPC's attitude to the MC must be Friendly or better. Works fine for me.I cannot for the life of me figure out how to add a guard to each of the mines? Everytime I try to transfer a guard, it turns them into a worker at the mine. Plz halp! Thanks.
However its much easier to just bring in guards from Fort Sera, if you have gained control of that place. Any random slave you hand over to the fort instantly becomes a potential guard no matter skill levels or attitude to the MC.