Sorry for the lack of a reply. I know imgur hosts only images lol, I'd probably have to use one service for images and another for videos. And yeah it'd have to use some kind of token or authentication and the host would have to allow private content. Guessing that doesn't exist, but just wanted to check.
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.