1. No rules I can think of.
2. I usually start out by creating a Sim. I might move them into a cheap house or apartment, or build them a house. Once they are established in their career, I try to have them meet someone - either another Sim I have made, or a townie (depending on looks and how compatible they are). From there, I usually work towards having them marry and starting a family, etc.
3. Maybe a few days? I will play with one Sim until I am satisfied with where they are in their life, or I get bored, or I wish to play with some other Sim or Sim family. Usually I have goals I work toward - like career advancements, getting a spouse, having a baby, etc., and then I move on.
4. I will use any neighborhood, depending on what lots are available.
5. The only drawbacks are with what the game does when you are not playing the other households. This was not an issue in Sims1 or Sims2, but in Sims3 I returned to a household to find that my Sim had married some random guy! For Sims4, I have aging set only for the current household and I will say it is frustrating when I return to a certain family after a while and have to re-establish friendships or romantic relationships because they have been "lost" over time. I wish time could freeze for my Sims I am not currently playing.
6. I prefer whatever I am in the mood for, so I do have a variety to choose from. I have singles, couples, families with toddlers, kids, teens... it all depends on my mood.