This is the crux of the debate right here; can you quote anything that would explain why 'All other Supplements are free to do so'? Are we still referring to the rule that says [Supplement] can Ally as Battle Brothers with [Codex]? Because the whole point of this argument was what that rule means. If it's a matter of Codex trumping BRB, don't all of those supplements also state that they count as their parent's codex, forcing the 'Same Faction' rules for Allied Detachments right back into play?There are a few restrictions on Allied Detachments due to the outdating of 6th edition wording by 7th edition army composition - Iyanden, Waaagh! Ghazghkull and (unconfirmed) Heroes of Fenris cannot ally with their parent Codex. All other Supplements are free to do so. There is a workaround, in that you can still bring a Combined Arms Detachment of Iyanden alongside your Combined Arms Detachment of Eldar.