It has often been observed that the larger the tank, the easier it is to maintain on a relative basis. The fact is that guppy breeders often install drip systems in their fish room, where a pipe slowly drips water into a tank with a drain pipe in it. So water quality is not an issue for this type of setup. Almost all guppy breeders keep their tanks bare to facilitate cleaning and to give pathogens no place to hide. A bare tank can also support more fish per gallon.
The necessity for smaller tanks is really driven by the need to segregate guppies so the breeder can practice selective breeding. Smaller tanks may require more frequent changes if you stock them with guppies to the same level as larger tanks. And they may be more awkward to clean. But your ability to support a variety of strains in your fish room is directly related to how many tanks you have. Two twenty gallon tanks cannot support a selective breeding program. For about the same amount of water (40 gallons), you can have two ten gallon tanks and four 5.5 gallon tanks, six tanks. Six tanks is the minimum required to selectively breed a strain.
In our experience, it is just as easy to foul a twenty gallon tank as a 5.5 gallon tank. Following the rule 1 inch (2.5 cm) of fish to 1 gallon is not a bad practice until you learn how to support more fish per gallon. You would not include the guppy’s tail in the calculation, since body mass is the important factor. (A new tank cannot support more than a couple of fish because the biological cycle has not developed.)
The shape of the tank does affect the stocking ratio. Tall twenty gallon tanks are inferior to short twenty gallon tanks. Water circulates from the bottom of the tank to the top of the tank faster in a shallow tank. This means more oxygen enters the water, and more carbon dioxide leaves the water. Oxygen is a limiting factor in how many fish can be stocked in the tank. The fish need oxygen for metabolism. Just as important, the beneficial bacteria breaking down waste also thrive on oxygen.
According to a scientific study (“Population Control in Guppies,” S Meryl Rose, American Midland Naturalist, Vol. 62, No. 2 (Oct., 1959), pp. 474-481), quoting an earlier study by Breder and Coates (1932), a 5.5 liter aquarium will support a maximum of 9 adult fish. “This was their final number whether they started with one or 50. Once nine had become adult, even though more wore born, no more could survive to adulthood.” They go on to say, “The data presented in the present paper indicate that both the production and survival of guppies decrease as the number of adults increase.”