For responsive comments, add the following to your CSS:
.fb-comments, .fb-comments iframe[style] {width: 100% !important;}That fix will also be in the v5.0.1 release we're planning to have out next week.
To assign the comments to only specific articles, you can use the "Include" and "Exclude" boxes at the bottom of the configuration area to really customize things. So, you can include every category, but then exclude specific articles.. or, only include the article IDs you want the box to show on. It's up to and should be as flexible as you're looking for, but if not, let us know.
Thanks,
Alex