Coral safe yes as far as i know. My one never touches my coral. As any Aussies would of maybe heard we had a huge storm a week ago and where blacked out for 5 days and my main SW tank got fairly destroyed so not sure if my mantis is even alive anymore.
I wouldnt trust them with any snails/crabs/shrimps ect. Sea urchins may be ok because of how tough they are i had/maybe still have one a similar size to my mantis and he never touched it. Ide say a big mantis would smash a small one though.
yes and no. depending on which specie you get, size, and if your willing to take the risk that comes with these guys... such as having your clean up crew killed
i would definetly say they are coral safe, and if you really want to be on the safe side you can get a spearer with that said i've read that smashers are more active