I usually start a save with two sims. I play favourites over and over but mix them with different partners in different saves. As for the town, I totally annihilate all townies and populate towns with sims I have made, sims that have been born in older saves and heavily tweaked townies from different worlds. I like to watch the genetics in my active household and that of the town over several generations. I spend an awful lot of time in CAS because I dress and modify traits of sims born in my town. There's usually several birthdays each sim day and I make sure everyone is dressed well and in their favourite colours. After a few generations, when I get a popup that a child has aged to toddler, I immediately take them into CAS and try to guess which of my original townies the sim is descended from before aging them up for a sneak peek :p