A quick google did not turn up any useful info on electric fence voltages for small dogs, but I suspect they probably *are* a little more vulnerable than large dogs, I just don't know how much. One thing you could do is to phone some fence charger manufacturers and see if they have any advice, tell them you want to use their charger for a small dog and want to know what is a safe voltage.
You will want a GOOD, DIGITAL fence tester, yes they're more expensive than the little four-neon-lights jobbies but a digital fenc tester will be *accurate* whereas the others are notoriously inaccurate. Fence voltages for dogs should typically be in the 4,000-5,000v range.
The ideal situation, if you are worried about the small dogs (and as I say, while I do not know for a fact whether this is warranted, I think it is a reasonable concern to investigate), would be to see if you can get a resistor or something like that, that will step down the voltage on one or more wires to maybe 2,000-3,000 volts or whatever you deem safe for the small dogs. Use that voltage on the small-dog-level wires, and the higher voltage on a lab-nose-height wire. Or something like that. I am not sure exactly what you want or where you would get one but I *know* they exist b/c I know someone who had that arrangement to lower the voltage on a stock fence that was charged pretty high elsewhere but they wanted it less strong where it ran around the house and yard. Ask an electrician or electrical-supply store?
Good luck, have fun,
Pat
