So.

Dark Eldar aren't incapable of being psykers. All Eldar are still innate psykers, the ability of Dark Eldar to manipulate their psychic potential has atrophied over thousands of years of negligence and culling, but, their ability to 'feed' off the pain and suffering of others is still only possible 'because' the entire eldar race has 'some' potential. The sheet answers why she wasn't murdered at birth because she wasn't born in the dark city, she was born as an exile, and the Drukhari's realspace/minor webway ports make the Dark City look like a bastion of law and order. I do specifically mention this a few times in the sheet.

Secondly. Dark Eldar have a fairly well-established history of fighting alongside all sorts, there are both Dark Heresy and Black Crusade official adventures which feature them. It's not that they enjoy doing it, or are unaware of the risks, it's that a combination of greed, a need to survive, and ambition push them to do so. That and there are specific unrevealed plot points which they are working towards, but that would be spoilers.

I do hope that clears things up, if you require me to cite sources, I can.