There's an aquarium calculator at aqadvisor.com. Though it isn't fool-proof and requires further research, it gives you good idea of what stock you can have in your tank. You enter your aquarium size, filter type and then the fish you want in there. The calculator will tell you if your tank can handle it.
Pygmy cories are pretty small and they swim mid-level (unlike most bottom dwelling cories)- you could have a small school of 6, I believe, with a snail. The next size up for corys are about 2 to 2 and a half inches- these would be your panda corys or bandit corys. Your tank could probably take a small school of 4- check the aquarium calculator. Juliis, I believe, get too big. Alternatively, you could put 6 otos in there- but I recommend an established, cycled tank for them because they are very sensitive when first purchased due to the way they are wild-caught. Never have any less than 4 otos if you do get them- they need groups to do well (as with the corys).
You'll notice from my sig that my 10g is pretty heavily stocked, but this is because it's a very heaviliy planted NPT, and these types of tanks can handle more bioload. I would not
recommend that stocking level for a non-NPT tank. :)