Unless your friend has a radically different style of play that I've seen before, he'll probably be playing gunline Tau early in the game, with most of his forces massed in one fairly compact area. Due to the Interceptor, you have to weigh the pros and cons of my recommendation, but I can tell you that it will be VERY effective if you pull it off.
Since you're taking Cast of Players, the Mask of Secrets Shadowseer and the Death Jester give you two potent Leadership penalty tools which you can buff even further with your Dark Eldar. You'll need to change your list slightly though as you'll need both DE HQs (and you'll need two) to join the Cast of Players in their Raider. You can keep your Archon as is and add either a Succubus or Haemie and give them the Archangel of Pain. You can WWP down in the Raider and when you open the Archangel of Pain, every unit within six inches will be taking casualties based off a Leadership check at -6/-4 depending on how far away they are from the hull of the Raider. The beauty of the Archangel is that it's an AOE attack and can hit multiple units within its specified radius. Then you get to shoot with the Death Jester and if you kill a model, the unit must take a fallback move at an additional -2. AND THEN, you get to shoot the Shadowseer's Grenade Launcher which can cause a Pinning Check and further casualties based on how much the targeted unit failed the check by. AND THEN, you have end of phase Leadership checks to see if the Tau fall back if they lost 25%, which will also be made at -4/-2. I've seen this cripple Eldar armies in one turn. Oh, I forgot to mention that your Shadowseer should also have Psychic Shriek if you roll on Telepathy (which you should).
As I said, you'll have to change up your list a little to try this out, but I've used it in several games and it is essentially like dropping a nuclear bomb on your opponent. However, I've never had to worry about Interceptor so might be worth putting Night Shields on your Raider to afford some extra protection on this very devastating unit.
EDIT: I highly recommend reading Jimsolo's Freakshow Tactics series of articles if you haven't already. Seeing as how you're playing DE + Harlequins, I think you'll find it very useful.