Additional Query String in a Node Url
All is right what you are doing.

You cannot pass additional parts randomly with URL without adding them to menu registry and associating with callback.

Drupal uses $_GET['q'] for default URL path and makes it SEO friendly. $_GET['q'] can contain 9 parts in total in Drupal 7 (7 parts in Drupal 6). Each part can be accessed via arg(n) function.