Generally tetras of NEARLY any sort ar pretty non aggressive. Barbs and "Sharks" tend to be fairly aggressive to other fish regardless. But as a rule of thumb, never ever have fish so small that they can fit into another fishes mouth, they WILL disapear! livebeares(guppies, platys, etc. they can tell you at the store) are pretty calm, but they will breed if there is a male and female so watch out for babies, they're a tasty snack for other aquarium mates.