They usually come in LR and no they are not good unless they are in a species only tank. There are two types of mantis, the smashers, and the spearers. Spearers eat fish and will most likely kill off everything in the tank if given enough time. Smashers eat snails, shrimp, etc. Anything with a hard shell. They mostly come out at night so it is hard to know if you have one, but they will dig burrows so you can usually hear a clicking sound which comes from them chipping at rock. They should be removed from a reef tank as soon as possible or they could start killing things. Also, be aware that they can split your finger open if you were to reach in and try to grab it, so be very cautious. There are several sites on the net about catching mantis shrimp and how to go about it. There are probably several threads about it on this board as well. So just do some searching.
And with all that said, I now, more than ever, want a mantis. If only I had somewhere to put another tank.