Why do these fish get along?

while they're all placid fish, the mollies shouldn't be in the same tank as tetra's as you know they're from diverse types of water conditions, mollies require sodium chloride to reduce the toxicity of nitrate, while tetras coming from a more acidic water have a greater tolerance for nitrate, given that your at the upper end of your loading of fish one would suspect that the nitrate levels will be towards the top end of the tolerance by the mollies and are will be struggling with internal nitrate toxicity which will eventually cause a breakdown in the blood structure and/or reduce there immune system.