I just added a site on disqus to which I log in with my google account. Take note that the username will be what is displayed in upper right corner of disqus.com dashboard, not your google account. The password will not be for google account either but for the disqus account (separate password). If you forgot it when signing in with google, like I did…, you can reset it on disqus.com.
So comments with the plugin and google account is definitely doable.
