Hi, I used the cas.fulleditmode cheat to change my butler, maid and nanny. I changed pretty much everything...their sex, age, appearance and even their traits and ambitions. I even renamed them. I only have 2 households that uses these employees. My second household hired these services today and the butler, nanny and maid that showed up are the exact same ones. The game isn't randomly generating different NPCs. Any help would be greatly appreciated. 
@siennablaise I've had one save where I rotated weekly between 3 households and the game spawned 3 different nannies. I didn't use maids and butlers in that save.
In another save, the game spawned multiple butlers. I don't remember about the nannies. I hardly ever use maids unless there are vampires in the household.
In my current save, I'm on the 8th generation and 4rd nanny after the first two retired and I got the 3rd one "fired." I've had the same butler for 8 generations, and I adore him.
These 3 types of service NPCs are immortal, so my sim turned the 3rd nanny into a vampire which caused her to lose her nanny job.
So, in my experience, hiring a nanny or butler hasn't resulted in the simultaneous spawning of a pool of available service NPCs.
I don't know, though, whether my experience is universal or not.
