Well, I was waiting to see if anyone with experience with penguin tetras was going to reply, but it doesn't seem that that's happening. I have neons in my tank, and have had them in the past with black skirt as well as blueberry and strawberry tetras (for those who are concerned, these were apparently the bred color variations of white skirt tetras), and never saw any nipped fins. I've never had penguin tetras, so I have no idea what their behavior is like.