The guys at the The Fish Gallery told me that a peacock would be fine in a 10 gallon.. said:
Absolutely not. A "peacock" (O. scyllarus) is way too large for a 10 gallon. Peacock is a general name for colored mantis shrimps but usually the O. scyllarus is the one that is called a peacock. IMO a 29 is pushing it. I wouldn't keep one in anything smaller than that. You have to include also that you will need a deep, rubbly sandbed which will take up a lot of your water volume. 40 gallon breeders are perfect for O. scyllarus. That's not saying that you couldn't keep a nice mantis in a 29 or even a 10 but you need to research which mantis you are getting before you buy one. There are many different species and different requirements for all of them.
This is a good place to start,
http://www.ucmp.berkeley.edu/arthropoda/crustacea/malacostraca/eumalacostraca/royslist/index.html#directory
Good luck!
Edit: Another thing I forgot to add was that O. scyllarus, and a few other species, are prone to shell rot which can be caused by too much lighting. Keeping corals with certain species of stomatopods isn't a good idea. There are several though that will tolerate high lighting. The "peacock" is not one of them.