I should update it at some point, here's some answers to tide you over though :)

1. You could create a single forum called "animals", and add the various tags to it (#mammals, #birds, #reptiles, etc). This would make the main view of the forum a combination of all of these tags, and then you could click on the tags in the forum header to filter down to a specific. You can't go any further in depth at that point though, so if you wanted a "subforum" for bird (with different child tags), that would have to be a separate forum.
2. Yep, you can create an unlimited number of them, but each one is independent of one another, and has it's own settings/upgrade path.
3. Correct - that's built into the steem blockchain and I can't change it. The only difference between Steemit.com and chainBB.com is the beneficiary rewards on chainBB (totalling 15% between chainBB + forum owners).
4. Sure - you could create a specific account for a forum. I have the @webdev account as the owner of /f/webdev. I'm planning on using the community features someday with the @webdev account on top of forums.
5. You can use whatever tags you want, they could be the same as steemit or you could make up new ones. There's only one pool of content (the steem blockchain), so any tags used on either will appear on both.

Hope that helps!