Custom RSS Feed Overwrites Permalink

You can use feed/photos with add_feed. This rule will prevent the overwrite permalinks issue. Change your function like below and flush rewrite rules.

add_action( 'init', 'add_custom_feed' );
function add_custom_feed() {
  add_feed( 'feed/photos', 'render_photos_feed');
}
function render_photos_feed() {
  //WP_Query for attachment
  header("Content-type: text/xml"); // Added for XML output
  get_template_part( 'rss', 'photos' );
}