Agree, the fish were not compatible in a 10g, plus I certainly suspect ammonia or nitrite poisoning. You didn't mention live plants, so presumably there were none, so that meant the 6 initial fish were producing ammonia and the bacteria just couldn't keep up. It had to establish first, and this takes time. Ammonia harms the fish (even if they live through it, the internal damage is done), then the nitrite.
The sole tetra may or may not survive, but please don't add any more fish until this is all resolved, and you have thought through the fish you want (that will work). So I will now turn to that.
"Compatibility" is many-faceted. Some fish are shoaling and must have several in the group. Glofish are shoaling, as are tetra; no less than 6 of a species, more is always better. However, a 10g has insufficient space for a group of Black Widow Tetra
[sometimes called Black Skirt, and there is also a "white" skirt, same species] so with the one you have you now have a problem. You haven't room for a group, and one alone is bound to be stressed and this usually brings on increased aggression, so it will turn nasty to other fish. If it doesn't survive it will actually be a blessing.
Second, fish have differing needs respecting space, water parameters, and the environment which includes water flow from filters, plants, wood, rock, decor, etc. Taking just water parameters, the molly must have medium hard or harder water with a basic pH. All livebearers (Platy
, molly, endler) need this, but the molly especially. Tetra are soft water, though the BWT can manage in medium hard water. The Glofish the same. Do you know the GH (general hardness) and pH of your tap water?
A 10g tank is very limited space. Depending upon the water parameters, we can consider possible fish that will live in this tank. BTW, you notice that the fish names above shaded, meaning you can click them to see that species' profile. That will give you data on their needs and issues.