I don't think a 1 person Civ should be able to close their borders.
Yeah, that's silly. I think a min should be 4
I think if it *is* possible for a one-person civ to have closed borders, there shouldn't be a cap on the total number of civs. Otherwise you'll get a majority of very small, closed-border civs and only two or three big ones for everyone else to join. (As, indeed, is often the case now.) If the game is designed to allow small civs, there should be a large number of them, not just 10.