My peacock lived with a Fiji Devil damsel for a week or so, and then one morning, I found the damsel dead and the mantis feasting on it. Don't know if Gojira took it out or not, during the night, but otherwise, I can't really imagine why the damsel died. Right now I've got a conch and an urchin in the tank, so I'll watch to see how they do, but no crab, large or small (including large hermits in Tulip snail shells) or snails (including golf-ball sized Turbos) have made it, though the larger tankmates have lasted several weeks before being eaten. One predictor of tankmate survival is how well fed the mantis is - if well fed, larger hermits and snails survive for several weeks - if regular meals (small hermits and snails) are not provided, anything goes (into the mantis' mouth, that is).
Ken