well ... it depends on the species. While I personally wouldn't put an O. scyllarus in a tank smaller than 20g, there are *many* smaller species of stomatopods that would do just fine in a 2.5g tank. Finding them, however, might be difficult -- the best bet would be to see if other reefkeepers find small hitchhiking mantis shrimps in their fancy uncured live rock.
I know that Dr. Caldwell keeps many small stomatopod species in pint and quart styrofoam containers in his lab. Of course, he also has graduate student labor to help with all of the water changes.