Change RSS format for specific categories

It certainly is! WordPress has filters called the_excerpt_rss and the_content_feed which filter the posts’ content and excerpts for the RSS feed. The filter is called in the loop, so you can use get_the_ID() and get_the_content() (and other loop functions) to fetch information about the post.

add_filter( 'the_excerpt_rss', 'wpse151603_rss_post_excerpt' );

function wpse151603_rss_post_excerpt( $excerpt ) {
    $content = get_the_content();

    if ( strlen( $content ) < 200 ) {
        $excerpt = get_the_content(); // Get full content for use in feed
    }

    return $excerpt;
}