Populate towns with own sim

@Cerulean0987   This is a very good question, and I hope you will get many tips.

I don't think there is an in-game way of pulling sims from the Library. There is, at least one, mod that can do it; called MCCC. I cannot link mods in this forum, so search by its name, and ask questions about its functions in the proper sites for it, or on The Sims official forum, where the ability to discuss mods are less limited.

The way I control the population in the worlds, is by creating my own NPC's, and giving them their 'jobs' with cheats. I also play rotationally, and place out sims in every house myself, because I want it that way. I have turned off the setting of filling up houses in: Game options>Gameplay>(uncheck) Fill Empty Homes; and don't have aging on. I make sure to place any sim I care to stay in the world in the tab My Households in Manage Households view. I haven't played enough to have tested this thouroughly, but I guess one way of controlling the population of service sims, is to create, for instance a bartender, before visiting a bar; so as to hopefully bypass the game doing so.