So, following on from Paul's suggestion on the 20-people thread, here is a new thread.
First, about our situation. We live in inner-city London in a 5-bedroom terraced house, built in about 1900, which we've owned for 20 years or so. When we first bought it, we needed the rent from lodgers to be able to afford the mortgage payments. Now, that's not so much of an issue, but we like the company so we keep the house full. Besides, it's a much more efficient use of space, heat etc.
Three out of five of the rooms have tiny ensuite showers, which removes one shared-living bone of contention. We eat our meals together and share all the food and drink. We don't have rules, as such, but a few principles: if you cook, you cook for whoever is hungry. If you make tea, make a pot. Being in London, everyone works and pays rent. We pay a cleaner to come in for two hours twice a week to clean the shared space.
Mostly, our lodgers stay for two or three years, and then move on - usually, they couple up with someone and start nesting. We've got a cohort of "grandchildren" as a result (we don't have children ourselves). Mostly, they move out to live close by, and we keep on seeing them; we've got a large community of friends as a result. Quite a few of them have started similar shared houses. One consequence of this is that a bunch of us often rent a big house for a week or two in the country for a holiday. Because everyone is used to doing stuff together, it just works. People who don't know us are amazed that we can fix up a big meal for twenty or so without dramas. But that twenty or so is for a holiday, over in two weeks, and there is usually a small team of earth-mothers managing things behind the scenes while the men drink beer. So probably not sustainable for much longer...
Do we have rows? Heck yes. But they blow over; we sit down round the kitchen table and talk about them. Moods, strops, whatever are part of the human condition; no one is ever blamed for saying bad things during a row. We've never had to ask anyone to leave; if anyone doesn't fit in, they will make their own mind up to leave. This is an advantage of the landlord/tenant relationship; it could get a lot stickier if everyone had an equal stake.
The size of our household is determined by the size of the house. We've always been between four and six people; five is better than four, and six is better than five but with six we don't have a guest room. I've often thought I'd like to buy the house next door and knock the two of them together to make a ten-bedroomed house, but there are two problems: it's not for sale, and if it were, we couldn't afford it. It would be good to have children in the house, too: it's a great arrangement for sharing childcare. The dynamic always changes, as different characters come and go. Twenty years ago, we were our lodgers' peers; now, we are Mum and Dad to a bunch of twenty-somethings. Ideally, I think there should be a full range of ages - if everyone is the same age, it's a frat house or an eventide home. But inevitably the economics of our present situation mean that we have younger people living with us.