Moray is correct. There is no freshwater mantis shrimp and you will get alot more replies if you posted in the saltwater invertebrates section.
Unfortunately, you aren't going to get a definitive answer as to whether or not they should be kept in a reef or with other fish, etc. There is alot of opinions out there in regards to them and how they should be housed. The best advice would probably be to have a spare tank on standby and if you start to see fish go missing then transfer it to it's own tank. Due to the fact that these guys are capable of smashing glass however, I would personally just stick with keeping them species only in a tank I'm not going to miss.
Other than that, the only thing I can think to add is if you are going to obtain one then don't join the multitude of bad stomopod owners out there who decide to flush them down the toilet or dispose of them in any number of other cruel ways because they get sick of them killing fish/smashing tanks/etc or grow bored with them (I don't know how they could grow bored with them as they are facinating little critters). They have evolved in a specialised manner and thus they deserve specialised care and the same respect as every other living thing kept in a tank regardless of the stigmas attached to them, they cannot help what they are/do.
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