I'm hoping there'll be Dark Eldar in Black Crusade. A friend of mine even speculated on how they might deal with Corruption: Dark Eldar sustain their souls and bodies using the psychic energy of the pain and terror and suffering of their victims. However, it is noted that it can be difficult to stir and Archon's heart with only the mundane acts of sadism and depravity, they require truly exquisite feats of artistic misery. Perhaps as Dark Eldar accumulate Corruption, they become jaded to the lesser and more common pains and anguishes of their victims, and their appetite increased dramatically, either for victims in bulk, or for the finest execution in feats of exquisite bloodshed and torture. In effect, they begin to develop a tolerance.
Dark Eldar and Chaos goals aren't mutually exclusive. The xenos are (probably) not capable of becoming Daemon Princes, and likely wouldn't wish to if they could. As for launching a Black Crusade, the Dark Eldar have no interest in capturing or holding territory, and no particular vendetta against the Imperium. No, the Dark Eldar seek two things; perfection in their chosen crafts of death and torture, and prestige. A Dark Eldar's endgame would involve besting the legendary Lelith Hespreux in the gladiatorial arenas, creating a masterful stitchwork abomination from the most fearsom xenos species and revered heroes in the sector, or carrying out a raid of such daring, cunning, power, and masterful execution that the towers of Commorraugh shudder with vicarious delight (a successful raid on the heart of Scintilla would be a good example, even better if one captures the governor).
Of course, Dark Eldar are complete and utter narcissists. They would fully expect to be stabbed in the back by their former allies, that is the way of life in the Dark City. This is the same for the servants of Chaos, but as has been previously stated, evil does not have to be stupid. Everyone who's read the evil-overlord list knows that its not a good idea to be a jerk for the sake of being a jerk. Why betray a friend now when they can yet be useful in the future, why dispose of a useful tool? Infighting is almost a certainty, but it can be avoided for a time, delayed almost indefinitely, until one side believes that there is more to be gained from conflict than cooperation. A good strategy to preserves such alliances is to make one invaluable to one's allies.