1. I do not really have rules beyond what fits with my story. For example, my vampire bartender that is too busy for kids is not suddenly going to become a teacher. I also try to maintain family bonds and develop a story to the world. Even minor characters need a little flavor.
2. Me, I usually start with parents and kids or a group of siblings. Then slowly marry them off and start their own families. Like I said previously, part of my goal is to maintain these bonds as they grow into an expanded and more free world.
3. This is complicated, because you try the formula out and either it sticks or it doesn't with that go through. The characters themselves I usually have saved in my library. My first big save like this I took over all of Newcrest and cannot bring myself to delete it to this day. I updated it regularly for nearly three to four years with new pack content and I may still go back. My other ones... If it sticks it can easily keep me preoccupied for three months. I save a lot of stuff in the library so the saves are not so vital anymore.
4. Usually, no. I have to be in one of those moods and have a grand plan to take over Newcrest. That three to four year save I did and I saved all those lots to reuse, but this is not typical. I love the space of Newcrest, but the neighborhood itself is boring and some of the lots are super small. I only recommend it if you do have those grand ambitions because you can get bogged down.
5. Just if you are trying to force it. I fell into this play naturally (when I do it) because I cared about the characters. I wanted to see them grow into houses with spouses and kids. Some I created parents or what not for later for some backstory. If you do not feel it for the characters it is just a chore, but if you do it comes naturally.
6. I usually play with large families. Either parents with a bunch of kids or or siblings and their pets. I have several families where I have a Sim in every age group and then matching pets. A lot of times I just play this unit, but when I do rotational I break up my large family after playing with the original unit and exploring where their lives go. Sure this is family play, but it also has elements of single Sims getting back to the large family stage and all the steps in between. I also have a penchant to make them occult families normally, but I do not break up my occult families so often. Rotational play let's me shake up my normal play by letting the family grow up.