As far as serpae nipping, I had a group of hi fin serpae that were just gorgeous. But every once in awhile they would go on a tear and chew everyone up. Little green neons disappeared. I took them out of the community. I worried about fin rot. I finally tried them again. In the moving a couple died, Eventually one died that seemed to be the terror of the group. Way over the top past normal Alpha behavior I think. After he died my serpae became the gentle men and ladies of the tank. Only one is left now. But as a result of this experience, I believe just one crazed Nazi can wreck havok. Still, perhaps some species of Tetra produce more crazy tormentors than others.
I don't have experience with the black skirts personally, but they do have the reputation of being more aggressive than some other species.