Mollies need hard water with a high pH (GH above 250ppm, pH above 7.0).
Congo tetras and most other tetras come from soft water with a low pH (GH below 100ppm, pH below 7.0).
The 2 fish are incompatible due to the difference in water chemistry required by the mollies. If you keep mollies in soft or acid water, they die prematurely. If you put Congo tetras in hard water, they also die prematurely due to calcium build up in their kidneys.
A better option would be rasboras or barbs. Black ruby barbs are small and peaceful but can hold their own with Congo tetras.