RSS Feed Confusion

Hi there,

I have some confusion over my RSS feeds.

I see that WordPress has the RSS feeds capability on it already and the feeds are like http://www.pokerblackbelt.co.uk/feed/

I did put the plugin “From RSS” onto the site which says

Adds ?source=rss to your permalinks from RSS, and 301 redirects them to the correct URL while dropping a cookie, giving you the opportunity to treat your RSS readers differently. Use from_rss(); to check whether a user came from an RSS feed.

If I use the plugin I get the following error at the top of the page if I am coming from a site using the feed

Warning: Cannot modify header information - headers already sent by (output started at /home/pokerbla/public_html/wp-content/themes/multi-color/single.php:6) in /home/pokerbla/public_html/wp-content/plugins/from-rss/from-rss.php  on line 28

Warning: Cannot modify header information - headers already sent by (output started at /home/pokerbla/public_html/wp-content/themes/multi-color/single.php:6) in /home/pokerbla/public_html/wp-includes/pluggable.php on line 868

If I deactivate the plugin the warning disappears.

What am I doing wrong?

Is it better to have the ?source=rss at the end of your URL? If I remove the plugin permanently will the feeds already out there be lost SEO wise?

Cheers