This is possible but you have to be careful on which species of tetra because not all can manage long-term with the warmer water that the common/blue ram must have to be able to properly function. Minimum 80F, preferably 82F is necessary or the rams simply will not live their normal lifespan (around 4-5 years). Fish are ectotherms which means their metabolism is driven by the temperature, and their tolerance range is quite limited. Their ability to manage well with some variation over the sort term should not be taken as evidence they can manage long-term at less than preferred temperatures. At less than optimal temperatures they have to over-work to perform their daily normal functions and this weakens them and that in term makes them susceptible to other problems, hence the shorter lifespan.
To the tetras and other characins. The Rummynose Tetra (Hemigrammus bleheri) is often selected. Paracheirodon axelrodi (Cardinal Tetra) is another, though the close relative P. simulans (False or Green Neon) is actually an even better choice. But definitely not their other relative P. innesi (Neon Tetra) as this is a cooler water species. These species come to mind, and I might be able to track down others from my notes. If the temp is kept at 80F there will be a few more options, but you really do not want it cooler for the rams. The species mentioned here are all mid-low level which of course is the same area the rams live in. Some of the hatchetfish and pencilfish inhabit the upper levels and can manage with 80F.
