Depending on the body shapes, some tetras will school with others. Depends on the fish though. I can tell you from experience though that one large school of one type of fish looks much more impressive than a bunch of odds and ends. You don't have room for more than one school of fish in a 20g tank anyhow. Make sure you've read up on cycling a tank, especially with neons, they are very sensitive to water conditions. Take a peek at the link below or the freshwater stickies.