I think mantis get a bad rap.
I have a 6" peacock mantis and he completely ignores fish. If it doesnt have 6+legs and is crawling on the sand, he doesnt consider it food. I've seen the lawnmower blenny literally go inside of the mantis's lair and perch as a lookout while eating algae. Completely ignored him. I also have a marine betta in there with it, which has GOT to be one of the slowest, if not THE slowest fish I've ever seen. It literally hangs in one spot for 3 hours at a time without moving. Mantis (luckily) hasn't touched him or anything.
My roomate had one a while back too, and it was exactly the same way. We even went down to the beach and caught like 5 small pinfish to throw in with it to test if it would kill fish. ignored them.
I think any incedent involving a mantis shrimp (at least a "smasher" type) killing a fish is self-defence (clown trigger trying to eat it...mantis attacks back with no mercy). But who knows....maybe I just own the most chilled out mantis in the sea or something.