The various randomized NPCs spawn from a list that contains all of the options. The various "all male" and "all female" mods modify this list, and I think there are/were some that made all Stormcloak soldiers nords (its been a while, I forget what else were in the lists in vanilla) the same way.
If you wanted, you could open a mod like that, or just the base ESM files in something like Tes5edit and remove all of the NPC records from the lists to make a mod with what you want. Making the bandits at a certain location all a certain type would also be doable, but a little different, as you'd need to make a list for just that location and change the NPC spawn markers there to use that list and not a generic bandit or witch or whatever list.
At least I think so, its been a while and I might remember things slightly incorrect.