There's no rulebook, it depends on how the community wants to do it.

Take my thinking here with an EXTREME grain of salt. This is just me, and how this place would work in an ideal scenario in my head. I have no control over what people want to do, how they want to structure their own instances, anything like that. With all that pre-amble out of the way:

An instance should contain people that are generally on the same page. Maybe one instance is more techy, another more social activists, another more art & literature, and we've already seen that language-specific regional instances are doing well. They should maintain enough diversity that they don't become echo chambers or cliquey. They all have their own rules, and enforce them on their own users and visitors to their communities.

The communities on a particular instance should play to the strengths of their users. The communities should reflect the general vibe of the users. Users will, naturally, have more interests than their instance provides for and pull in stuff from other instances. In a sense, your instance and the communities in it are your home street, where you know most of the active users, and the others are the big city, where you go to get things you can't on your street.