I have a friend with 6 reef tanks set up and all of his tanks have mantis. 3 are 29 gallon biocubes and he said they have never bothered anything, he feeds them shrimp and the dudes are awesome to watch...I was amazed as I had heard for the last 8 years ive been in the hobby how aggressive they are yet what he was telling me was totally opposite of that.
Yeah, it's that they are predatory so of course if they are hungry they can grab a fish or a hermit etch... but look at what we do with pet cats and dogs... they can hunt and kill but we keep them in our homes and few of them turn on us.
But the pit bull got such a bad rep folks forget that they can be a great family dog if they were raised right and socialized right.