PC Robots.txt automatically creates a virtual robots.txt file for your blog. Your robots.txt file can be easily edited from the plugin settings page.
If a physical robots.txt file exists on your blog, WordPress won't process any request for one, so there will be no conflict.
No. That's the correct syntax for allowing a bot. Because there's nothing after the Disallow: bit, it's actually saying don't disallow the Googlebot anything, or in other words, allow it everything.