I think that's what mods are for. At least back in the day that's how it worked. Of course some mods will abuse their powers and just do stupid stuff but in the end that's part of what made it so fun. You had to iterate through dozens of forums and hope that yours had a fighting chance because the mods did their job properly.
Exactly that. I would explicitly allow it, esp since most have some interest in some of the topics. Even off-topic chat can be fine if it's in an extra category. The only thing to avoid is pure spam but with some threshold to sign up (e.g. verification) that can be kept relatively low.