It might help to review this page. It provides an explanation of what the different kinds of NPCs are. Scroll down to the "Who is in My Town?" section. The rules and restrictions that NPCs have and what we can do with them using which tools vary greatly depending upon the type of NPC we are talking about.
https://nraas.wikispaces.com/Population+-+Town+Management
Role Sims can be residents (with or without families) or homeless. They can be controlled with the Register mod to be pulled from existing unemployed residents, spawned as new sims, or manually assigned by the player.
Service Sims must be homeless and do not have families. The homeless "household" cannot handle marriages, pregnancies, babies, or those siring (fathering) children -- that is, such sims cannot remain homeless and therefore cannot remain with Service assignments. We can add sims to the Service Pool, but that never guarantees that they will be used as such. The Service Pool is not strictly controllable or stable.
The genetics of Role Sims depends on where they came from, were they homeless or resident, were they born in game, etc. The genetics of Service Sims (and incidentally NPC Roommates) can be controlled by way of NRaas StoryProgression in similar ways to but using options under a different menu from that of resident immigrants.
Does that help get you started? :)