1. You can play freely, but I'd suggest you to have a pattern like 3 days, 4 days or a whole sim week. Otherwise some households could progress further than the others. After picking the "time lenght" pattern try to write down the ones you already played so that you don't forget and progress someone more than the others by accident. I like to play each family a whole week, as each household can experience the season and holidays fully.
2. If you play premades/more than one world, DON'T MARRY MULTIPLE SIMS ACROSS MULTIPLE WORLDS. Thats unrealistic. Just don't. (I mean you can, but thats not realistic at all). You could like choose 1 sim to marry someone from other worlds, but you could randomize a dice or something to know who would do such thing. Some worlds don't have that many sims, BUT there's whole gallery full of maxis premades or other sims made by other people (if you don't play with premades). I'd suggest you to play with 8-10 households in each world. Place down gallery sims that match each pack i ntheir respective town and so on.
3. Also, if you play multiple worlds, you can play each family in each world a whole week, or you can play all families in a world for a whole season cycle (all 4 seasons) and then move to the next world.
4. I have a lot of little rules to play premade sims, so... I can share that later, but i don't want to elaborate much here. You can play however you want (just please don't marry all sims across worlds, thats weird).