I don't seem to have much of this problem. All my townies seem to be Sims that I have already added to the game.
I created my neighbourhoods first, filled all the houses with my favourite Sims families, rigorously got rid of all Sims that Maxis had put in their family gallery in game and found that my Sims - and some I allowed to stay, were filling the game roles of mixologist etc., Mrs Pancake turned up as a mixologist for example and all other roles taken by family members I hadn't found jobs yet. One new Sim made friends of all the Sims who past her gate when moving in and all of them were living in my houses.
This has come after the four years of playing the game, making dozens of families and houses and making sure that I evicted any Maxis families that seemed to find my builds irresistible. I move in one family and five move in around them without my permission? No way!
I have a limit of 200 Sims so I suppose that they do have to fill those roles when the limit is nearly reached by me.
