We want to be able to dictate whether an article requires a sign in or not, so we don't have multiple boards with different permissions. As in, some content we might deem public and some behind the sign in wall. Is there a way of doing this at article level at all? Using Labels or Tags?
No, not without heavy customization. Khoros doesn't have content-level permissioning. It's all done at node (board/category) levels.
Anything's possible, of course, but it really would require extensive customizations which you'd then have to test / support for every upgrade thereafter. You should weigh the overhead on that vs. just having the content on boards with the appropriate permissions.