I have done what you said- put in premade families that I either made myself or downloaded, then evicted them. That makes them become townies/NPCs (to an extent). Every time I open up my game, I go in and delete any auto generated townies, aside from my butler.
They walk around town, populate community lots, fill positions like bartenders, waiters, baristas, delivery people, and DJs. Their pets will even wander around town like auto generated strays normally would. There are some positions the game still generates new NPCs for though. The ones I've noticed are butlers, maids, repair people, and mail carriers.
The Sims I don't tend to see walking around or filling positions much are actually the ones I've put in houses.