Smaller smashing mantis shrimp actually can be kept with certain fish (usually), though yours does look very... big.
Rose is approx. 2" long, actually. She will be about 3" adult size. All the same, she can, will, and
has killed fish almost twice her size. I tried a couple of small damsels as tester fish to see if I could keep anything with her, but it was a no go. It took her a couple of days to get them, but eventually she did. The tank is too small to house a fish large enough not to be a prey item for her.
Here's a shot of her with my finger for size comparison:
No.
Both my mantis LOVE snails, they're actually one of the main prey items of a smasher type in the wild. The big
Odontodactylus scyllarus usually doesn't go after them because he is fed well and there isn't enough meat in them for it to be worth his while to break the shells, but if he goes without food for a couple of days he will start picking them off. I have to replenish my supply occasionally. So far the trochus have lasted the longest. Nassarius are relatively thin-shelled, so he picks them off too fast for it to be worth purchasing them, and turbos fall somewhere in between. There's a big conch that lives in that tank whose shell is too hard for him, though he spent a good hour trying to crack it the first day; I gotta have
something for the sandbed!! Hermits get picked off quickly with him, too, once they reach a certain size and have some meat on them.
As for little Rose the
O. havanensis, she loves smaller snails as well, but can't seem to crack larger trochus, and I think there's one very large nassarius who has survived. Hermits are ok with her if they are large-ish, living in thick shells, and don't get caught while changing homes!
So far all the echinoderms I have tried have been just fine, the big guy isn't the least bit interested in the brittle star (it even lived in his burrow with him for a time, until he smacked it when it tried to steal his krill and it moved to the other side of the tank), but he has eaten a small upside-down jellyfish, so while cnidaria are usually safe, I wouldn't go so far as to say
all are safe