A turtle shouldn't be housed in a 20G tank, unless it is tiny. A good rule of thumb to have is 10 gallons of room per 1" of shell, with double the tank filtration since turtles are very messy. For the smallest turtle species, I wouldn't go any less than 55G. For the common Red Eared Slider, you are looknig at a 100+G tank, since they grow very large.
Now, as to your question, it is possible that mollies and some species of tetras will nip. Mollies are also Brackish fish, and do best in hard water. Also, you will be overrun by fry eventually if you get a female, since they can store sperm for months. Some tetras will aswell, such as the more "energetic" ones, like Serpea or Black Skirts. I would only suggest, possibly Cards or Neons, but even those can be nippy.