The big problem with using schlappen for streamer wings is the stem is so soft that the wing will foul around the hook. The only ways to counteract this are: tie the wing shorter than the hook's bend; or tie the schlappen in at the bend of the hook as a pusher-style tail. Also, schlappen is too opague to simulate the translucency of baitfish.
Do yourself a favor and use quality saddle hackle or neck hackle for streamer wings. The flies will fish better, look better, and tyng will be easier. Keep in mind that the masters of featherwing streamers use saddle and neck hackle for a reason.