Is it possible to use LibGuides as a Library's intranet (not publicly viewable)?
Yes, it is. :)
We have several clients who requested that they be removed from the Community list and that their pages not be indexed by our search or search engines in general because their sites were only going to be used internally. We were easily able to accommodate that.
If you have some content you'd like widely available and some you only want to use internally, you'll want to check out CampusGuides / CommunityGuides. That system is LibGuides + more! One of the features is limiting access by IP range, so you can ensure that no one outside of those IPs will see that content. You can also password protect guides. See the Springshare website to find out more.