LOL...I didn't scroll down, I just clicked on the link. After about half the video, I thought to myself "I can't help feel sorry for the crab, but it's the circle of life."
Mantis will kill fish too. They're really efficient killing machines. Definitely best in their own, seperate tank.
They can do okay in sumps, provided it is lit and the mantis is sectioned off from equipment. I don't think it's the best idea, but it can be made to work.