Cloud, I would suggest looking through the past posts here in the forum, there is LOTs of info on your question!
I think the chances of you getting a mantis shrimp depends on where the rock is from, you have a better chance of getting on if the rock is from Tampa Bay, it seems. But then you have to consider if it's cured rock, was the (foreign - fiji, marshall, ect.) cured in with tampa rock? If so, a mantis might have migrated with it. In general I think you chances of getting a mantis in 22 pounds of rock are fairly slim.
As far as wether or not you want a mantis in your Reef tank, it seems general consensus would say they are a BADâ„¢ thing to have in the tank. They do exhibit some disturbing habits.
Each type of Mantis has it's drawbacks. I believe the smashers are the most commonly found in LR. Smashers will make a cracking noise in the tank because they are cracking open snails or hermit crabs, bludgeoning a crab, shrimp or fish to death, or breaking rock (I'm still not certain if the breaking rock thing is true). Some will catch and kill fish, some don't. Mantis like to live in tunnels or holes in the rock. They can dig and move sand like crazy.. When my mantis was settling in, he make sand storms in the tank because he was building his tunnel and redecorating. They can make a home in a hole in the rock or might burrow a tunnel in your substrate. If they can, they will move rock, plants, corals around to suit their taste.
If you get one, on purpose or not, I would set up a tank for him if you can. They are truly fascinating and intelligent creatures. If you do get one and do not want to keep it, let the people on this forum know and someone might want to give it a good home or call your LFS and ask them if they know of anyone looking for one!
For more information on Mantis Shrimp, check out:
The Lurker's Guide to Stomatopods.