A healthy fish should not be sucked up against a filter - providing that you don't have a filter rated for a pool on a 29g tank. If you are concerned (or paranoid), just get a filter sponge and attach it to the part that hangs into the tank. It is small fry and sick fish that usually succumb to a filter. It's called an HOB (Hang on Back) filter, by the way.
Ideally, you would want one bigger school of fish than two smaller ones. However, if you were to get 12 tetras, 6 neons and 6 glowlights, you'd be alright. Sometimes mine school together, sometimes they don't. In a 20g tank, they probably wont' do much schooling anyway once they get settled in. Fish that are not schooling are happy, contented and safe. Constantly schooling fish (except in the wild or in very large tanks) are showing that they dont' feel completely safe on their own.