You don't have to worry about adding in shadowseers. Check out page 41 in the dark eldar codex that discusses shadowseers. Pretty awesome! They're actually the only psykers allowed in Commorragh. They even often serve on Archons' councils.
Personally, I converted a death jester out of a one-winged scourge I had sitting in my gaming closet for such a long time. I came up with a story about him failing on his first flight out of the Haemonculus' lab. One of his wings was torn as he flew to the upper spires of the Dark City, and he plummeted down to the lower levels. Miraculously, as if someone (perhaps Cegorach) was watching over him, he survived as there was something that broke his fall. As it turned out, he had fallen onto a death jester. The scourge began to laugh uncontrollably. After regaining his composure, he donned the mask and shrieker cannon of the death jester that was crushed beneath him, found the troupe that his unwitting savior belonged to, and was accepted by them without any question of what had happened.