Good fish for a 2 gallon tank w/neon tetra friends already inhabiting?

The rule of thumb is 1" of fish per 5 gallons of water in a new tank. As the tank matures its 1" per gallon. I would not put anymore than the 3 neons you have in there or you are going to cause your Nitrites and Nitrates to climb and loose fish since Neons are delicate. The rule of how many fish to put in per gallon is due to the waste load. Too many fish in a small tank would be too much waste for any healthy bacteria to break down to safe levels. Therefore, causing your levels to go up and causing stress on the fish. Stress also brings on diseases and/or death.