The JFBConnect Authentication, System and User plugins are all required for JFBConnect to properly login and authenticate users from any social network, so leave those enabled.
As for the problem you're having, for the new app, the "Invalid Redirect URI", please make sure you've followed the
LinkedIn Application setup guide for Joomla
. Specifically, step 3-D-b for setting up your callback URLs.
If that's properly setup, then we'd need to see the page that's having problems to investigate further. If so, please post, or Private Message, a URL we can look at.
Thanks,
Alex