Thanks for asking G-S. Nothing against the type of relationships at all (personal view=each to their own choice as long as of consential age and no-one is hurt or abused in any way), but a Zelda or Gundam Wing fanfic would be a bit off-topic and out of place in a Harry Potter fanfic area. Unless, of course, you can incorporate some Harry Potter stuff in with the fic (like King Kong meets Godzilla), but I don't know anything about Zelda or Gundam Wing, so wouldn't know if that would be possible.
Regarding the rating, a subject line including the rating may be adequate, and possible editing of some words. It does depend on overall content though, so if you can do a HP meets GW or Zelda fanfic, it's possibly best to leave out any explicitly s*xual scenes.