WordPress.org

I installed the plugin but I see no changes in my RSS feed?

This is usually due to caching. If you serve your feed via Feedburner or a similar service, it may take several hours before you'll see the changes. Also, most RSS readers will cache the feed and not show the updates immediately.

Won't this break my feed/make it non-compliant with web standards?

Probably not. In my experience the modified feed looks fine in Google Reader, and the W3C Feed Validator also says it's valid.