1. The only real rule I abide by is switching households between 5 and 6 AM, so no one inconveniently comes home from work because you switched households.
2. At the moment I play five households with 14 sims in total. The dynamics within the households differ. I can't say I usually play with a specific set-up because there isn't one. Two of my households consist of one sim. One household is a mother and daughter and the two other households are parents with some of their kids.
3. I've been playing (and storytelling) with this save for months now, so I'd say it has kept my interest for quite long. I think it's the variety in household size and the different storylines that keep it interesting.
4. I've barely used Newcrest. I put a roller rink and a park there and that's it. I started playing with premade households so they already had houses. Most of my sims live in Windenburg because that's where they started out and I wanted my related sims to be close to each other. One sim lives in San Myshuno in an apartment but that's because his girlfriend (at the time) moved there and he moved in with her.
5. Depends on how long your rotations are. Mine are very short sometimes which means none of my households pay bills. I don't know why but they don't get them anymore. It's also harder to get a promotion with shorter rotations because you usually have to go to work twice with all your tasks completed so when I get back around to a sim they're a couple days older already. The coming home from work/school thing in the middle of the day whenever you switch is a real drawback too but avoidable by switching at a time when you know everyone in the other household is home anyway.
6. I started the save with a single sim and that's my go-to starting set-up but I fell into the Villareal family and it all went on from there. I do enjoy bigger households so you could say I'm into family-play but I'm also really happy when I can switch back to a single sim and focus on them.