The Dark Eldar hate Chaos, but a bit differently than their Craftworld kin. Even though their ancestors were responsible for the Fall and their hedonistic and self-indulgent lifestyle is very similar to that of Chaos they find giving into such powers to be incredibly weak. They are an arrogant people and don't take servitude well at all. Slaanesh is still "She Who Thirsts" to the Dark Eldar and practicing psychic powers in the Dark City is punishable by a very slow tormented death.
You could use the rules as a basis for the Crone World Eldar though. Perhaps Slaanesh "released" a few for service abroad and their ships (a blend of DE and CE technologies) are seen more commonly.
The whole "Dark Eldar despise Chaos" thing always seemed to be blown way out of proportion, imo. In editions past it's been commented on that they have served as mercenaries for Chaos forces, the price usually being slaves of course. The new Codex makes no mention of specific enmity against Chaos, aside from She Who Thirsts, which is a rather special case, since Slaanesh is actively trying to eat their souls. In fact all of the cited battles against Chaos (in the codex) seem for more pragmatic reasons than "hurr Chaos!" Like stopping them from breaking more of the WebWay, or.. because it's fun.
And Tome of Blood even flat out mentions that Dark Eldar like to travel to a specific world in the Vortex, the world of Kurse, to view the fighting pits there. Or to fight in them, even.
What I'm getting at here, is that the Dark Eldar don't really seem to care overly much about any othe race or faction in the galaxy, and in fact, couldn't care less. Unless it somehow pertains to their personal gratification. This is the same race that saw the galaxy-devouring horror of the Tyranids and thought, "I bet those would be great in a cage match".