If it were me (and I'm not saying I'm going to do this, I most assuredly do NOT need a fourth Skinnie force...) I would use some of the second-generation Zykhee, maybe the Deathdancers and Ka'Char, and I would use Militia heads to make them look like, well, Skinnies. I wouldn't worry at all about the second elbow on each arm, and putty the hands for two fingers each and extra splay toes on the feet. I would have to find a reasonable consistent piece I had multiples of for the Disruptor Glaive heads. There is a squatting Ka'Char model thatwould be perect to drop on the back of a critter for Tawn Riders, a few cheap monsters from the dollar store could fill in for those pretty well.
But really, I'm not going to do this myself. Honest.