Comments Link Optimization waht prevent all search engine crawl your comments link.
No. The robots.txt file is automatically created and modified, and does not modify existing content.
No. After you install and activate the plugin needs to do nothing.
You do not need to add nofollow to comments link, Because the plug-in by modifying the robots.txt file to prevent search engines crawling the comment link from your site. Robots.txt takes precedence over the nofollow.