The pair of kribs should do fine. I had a pair of kribs in a community tank that did great, even when breeding. I had a flower pot for them to breed in, and put it on one side of the tank. They would defend their breeding area, which was made smaller by having the flower pot near a wall of the tank. (Half their territory was outside the tank). The rest of the fish had plenty of room in the rest of the tank and the kribs had their little area. After a few weeks, their babies would disappear (I didn't net them out or try to raise them), and then another batch of fry would appear. When they weren't breeding, all the fish could be in all of the tank.
Also, the Colombian Tetra's are in too small of a school. If you had many more of them, they would not be so aggressive. They would play/spar amongst themselves and leave the other fish alone. But because you have so few, they don't have enough of their own kind to play with and will be aggressive with whatever fish you put them with.
Emily
