Those chemical bacteria starters can leave you with a tank that's prone to water chemistry swings. I wouldn't trust that stuff. Using something from a reliable, established tank will jump start the cycling process, but you'll need to add fish to the new tank very slowly. Once established, a tank less than 30 gallons could certainly work well with a couple of 50 percent water changes weekly. Remember, a tank with just a few small fish in it still needs most of the water changed regularly if you want to maintain good oxygen and mineral levels in the water.
"Fear not, my young apprentice. Just change the tank water."