If want you keep your tank at a low temp like 70, you might want to choose your tetras by their geography.
Tetras from Amazonia or northern South America, and/or southern Central America are going o appreciate water temps around the "high" 70s or low 80s.
If you choose tetras from Mexico like Astyanax, or tetras from southern countries of South America, like Argentina or Uruguay, you will have no trouble with those low 70s temps.
Another example of species that tolerate low temps well are Buenos Aires tetras, they take temps into the low 60s.
Although not tetras, Denson barbs also do well in temps in the low 70s, and high 60s.
When living in WI, I kept Uruguayan species, without heaters, without a problem.

 

It’s not that I want to keep the tank cooler than tropical fish levels, but that plants like temperatures cooler than most tetras.

 

I keep my shrimp tank heated to 25C with some ember tetra's.