There are not hierarchical permissions in NodeBB. If you would like to see hierarchical permissions, you could open an issue on our Github page and request such a feature.
I do believe we have a way of copying permissions between groups, so that could help you out in doing what you want.
Yes, indeed, if you're looking to scale up to handle high concurrent user loads, scaling out horizontally is your best bet. We've got a client right now serving 700+ connections at the moment, and they're on four separate NodeBB instances. It's probably too much for what they need, but it's always nice to have more capacity than needed in case of spikes, etc.