Mantis are a fascinating creature (obviously I'm a little biased). There are unfortunately a lot of myths about them on the internet. It's myth that the mantis shrimp that came with your live stock will destroy your system. In almost every case, rock hitchhikers are smasher as opposed to spearer type species who's max size is about 3 inches. Their primary prey are small crustaceans, snails and mollusks, not your prized Angelfish. Small hitchhiker stomatopods of 1" will take about 2 years to grow to any size threatening to any animals larger than hermits and snails, giving you plenty of opportunity to trap it. Smasher type mantis can grab a small fish but they would prefer hermits and snails. I know of a number of people who keep fish as well as soft corals in their mantis nano.
There are also no documented cases of receiving potentially "tank-breaking" O. Scyllarus, G. Chiragra, G. Platysoma or H. Californiensis as hitchhikers. Those are the only 4 species who can do it and they can only do it when they get larger. The O.s. (peacock mantis) is often available for sale mainly because they often leave their den and wander across sand flats where it is easier for divers to collect them.
Mine like frozen cocktail shrimp (thawed and rinsed well obviously) and formula one on occassion. I've never heard of a problem feeding a mantis shrimp unless it was overfed, ill or preparing to molt. Our pet currently gets pieces of tiger prawn, which divided came out to be about 20 good-sized meal chunks. I'm going to vary the diet using as much fresh seafood as I can including, shrimp, squid, scallops, clams, mussels, fish, etc. Maybe even a tiny bit of fresh Abalone. I have read of them being lazy after being fed lots of shelled prepared foods that they may hesitate to break open food if they can help it. One thing to note is that smashers will sometimes lose their "clubs" if they're not used over time, so it's a good idea if you can, to occasionally feed them whole clams, mussels, crabs or snails in the shell. Also freeze-dried krill are a great media to absorb liquid vitamin supplements. Something I've read that was really interesting was that in studies, Mantises would selectively choose food for the most food benefit that can be opened by the least number of strikes. It seems they're smart enough to plan their diet and energy usage.