In that size tank, with the Zebra Danios ("GloFish"), you could probably maintain, easily, 3-6 average Cory Cats. I would go with either Paleatus or Panda in an unheated glofish tank, as neither one requires "warm" water. Panda are a lot smaller, and a lot prettier.
Personally, I think I would go with Habrosus rather than pygmaeus. One of the troubles you'll get into with any cory cat is that they'll need to eat, and the danios are some of the most aggressive feeders out there. Pygmies like to eat in the water column, while Habrosus will get food at the bottom. You can target the habrosus with a sinking pellet, and the danios won't eat it all. You'll definitely need to feed the cories a sinking pellet, as they're not going to live on "leftovers." With either of these two small cories, you'll also need a heater (not sure if the basic set-up has one).
Rather than going with a Cory Cat at all, which is going to be hard to feed, I think I would choose a loricariid. You could put a trio of Whiptail Cats, or Farlowella Cats, or a pair of Ancistrus in there. You could even do 3-6 Oto Cats. They'll eat the algae, which is a bonus, and you'll feed them with either zucchini slices or algae tabs, which the danios should leave alone. Farlowella are really neat, unusual looking cats, and I think a 5-year old would get a kick out of them.
