I agree that a larger tank is better/more stable.
However, I have several smaller mantis in 5 hex and 6 gallon Eclipse tanks.
My G. viridis has been in a E6 for 3 years now, and he seems to be doing fine. As well, my G. platy's tank is the most 'healthy' looking, covered in coraline algae, and dotted with little feather dustersm pods, etc... It is possible to have a healthy small tank, and this from a relative newbie (My G. viridis was my first entrance to fishkeeping).
I do 1 gallon water changes every 7-10 days, not a big deal, just 4 dips of a quart jar, less than 5 minutes of time. I use the pads that the Eclipse come with, replacing them every 3 weeks to a month, as well as having 4-7 pounds in each tank of live Rock, and 2-3 inches of live sand. The G. platy has a red serpent star for a roomate, which means that I have no issue with troublesome leftovers, and none of my mantis seem to bother the small stars that live in their tanks. I would recommend little stars for garbage men.
I leave 1-3 hermits and 2-6 snails in each tank, replacing as needed.
-Uriel