OK one gallery, several albums for different clients. Is there a way to hide the thumbnails until the viewer is logged in so that they only ever see their own album thumbnails?

Unpublished items do not show unless you are logged in.

OK looks like I forgot to save when I changed the setting...but now there is no way for them to log in. How do I default it to a log in page?

Sorry, not sure what you are asking. If you want a login form on your pages, there is a plugin that will do that. If you want your gallery not to be viewable except to logged in visitors, set the gallery password.