"Tetra" can include dozens of species of characin, and most will get along but there are exceptions. Some can be real nasty, fin nipping and worse.
Tetra are shoaling fish, meaning they live in groups of hundreds of their species. They should be kept in groups in the aquarium, six is usually considered minimum for a species, though some need a few more (the "nippy" ones especially).
Most species will always show more interesting natural behaviours in larger groups; there is a real difference between say three or four and a group of eight or nine.
If your tank is a 20g, I would stay with one or at most two species, depending what they are. Some need room to swim, others are more sedate.
Byron.