Peter,
We have no problem with what you're looking to do. With JFBConnect, when a user 'connects' their Facebook account to an existing Joomla account, they can login with *either* account from that point forward. Additionally, with our upcoming v5.1 release, we'll be adding Google+ authentication with the same abilities.
Furthermore, we have a configuration option (which is enabled on this site) to automatically generate new users that login via Facebook or Google+ to automatically create them a password as well. If you check the "Welcome" email you received from us, you'll see there is a username *and* password for your account there. With that, you can use FB or those Joomla credentials to login.
If you want to test, you can try 'connecting' to Google+ by using the option in the My Account menu in the top right. If you do, you'll be able to then login to the site with Facebook, Google or your Joomla credentials.
Whew... I hope that all helps, but if you have any questions, just let me know. For a good overview of the features JFBConnect adds to JomSocial, see our features page below with a good demo video:
www.sourcecoast.com/joomla-facebook/jomsocial
If you have any questions, just let us know, and we'll gladly help answer them.
Thanks,
Alex