[Andy Peatling]

About private/hidden forums, will topics made in these forums show up on the main BP forums page for group members who are a part of those forums?

As it stands, no. This may change if I can work out a way that doesn’t require a slow query.

Also about bbPress plugins, would it be possible to somehow take advantage of bbPress’ existing plugins and use them in the BP forums environment?

Possibly if you place them in mu-plugins, but you don’t have the bbPress backend.

Or would we need to port some of these plugins over by adding the appropriate filters/actions that BP forums uses?

I would try and port some if you’re up for a challenge.