There seem to be an interesting rule where you CANNOT take an allied detachment that is your own faction.

If I ran the Dark Eldar Haemonculus Coven Codex as my primary army (to get those juicy warlock traits rolls instead of the medicore regular Dark Eldar table) I apparently cannot take an allied detachment of regular Dark Eldar of 1 HQ and 1 Troop but I MUST take a regular CAD of 1 HQ and 2 Troops.

Can I confirm this? Because Dark Eldar Warriors don't really fit in my army and I certainty do not want to take more than 1 of them. I am mainly taking the regular Dark Eldar for the scourges as Haemonculus Coven has basically ZERO Anti-Tank apart from tail weapons on the pain engines.

Any clarification would be helpful! (Also recommendations on Anti-Tank for pure Coven lists will be appreciated too.)