by Norbu the Destroyer » Mon Aug 05, 2013 10:12 am
I don't think there is any real ruling on this, it seems to fall under the "whatever the T.O. decides for the event".
Personally I don't think you should be able to. When you take supplement Iyaden....which codex are you using? Eldar. The supplement is only a couple pages, you need the original codex to play it. Well lets look at the ally matrix. Hey, no Iyaden, I should be using Codex Eldar as my "parent" codex to see who I can ally with. Now can Eldar ally with Eldar, most tournies do not allow it.
If it is allowed that the supplements can ally with their "parent" codex, I guess it would be equally likely they will allow codices to ally with themselves (Tyranids).
Who knows if there will ever be a real ruling on this, but it seems it would be up to the T.O. or discussed with your opponent before a pick up game.