I feed my mata a school of about 200 minoes that are a bit larger than tetras each week. That would be a terrible idea to mix them.
well my tank is pretty big,(1500 liters, 210x110x75 cm) so a turtle that maxsize on 10 inch (25cm) isn't too big i dont think... Or do they really need that much swimming area?10 inches is still quite large and these are quite aggressive turtles. I am not saying that there is no way for it to work but I would imagine you would need a pond size tank to house one successfully with fish.