So it sounds like you have a number of things going wrong here. When fish die, or any organic matter decomposes in your tank (poop, excess food, etc) it releases ammonia, not bacteria. Certain kinds of bacteria are actually very good for your tank and necessary to have a healthy tank. This is why we need to cycle tanks before adding fish. My guess is that's why all your fish died at once, because there was an ammonia spike. There are two kinds if bacteria: a kind that converts ammonia into nitrite, and a kind that converts nitrite into nitrate. The process of growing these bacteria is called cycling. (you will need to get the API freshwater master testkit in order to monitor this process). These bacteria are veeery important because they prevent fish from getting poisoned by their own poop. Also weekly 25-50% water changes with a gravel siphon are critical. Try reading this article on cycling:
I just learned about cycling but I already have fish. What now?! - Aquarium Advice And yes, nornally Petco and Petsmart employees don't know very much about these things. They generally just want you to spend your money buying things. So don't trust them- ask on this forum first
Also, as other people have already told you, a 10 gallon is a bit too small for a Molly. And tetras are schooling fish so they like to be in big groups. One thing that might help tangerine is if you add some more plants to create more hiding places for him though.
Edit: also, mollies are omnivores which means they eat both meat and plants. This is why domino tried to eat your floating plants. Having some vegetable matter in their diet is good for them, so try feeding him some boiled peas with the outside peel taken off and squished a little bit
my mollies always loved that. I've heard people say to feed them lettuce and steamed broccoli but my mollies have never showed any interest in those.