I've read a lot about modifying the one-touch units and am eager to try the 'foot-switch' one.

What I don't understand is what size of wire to use for it. Is it possible to have either too skinny a wire or too thick of a wire?

What I have readily available is some speaker wire - 18 gauge if I'm not mistaken. It is 2 wire strands in separate insulation sheaths, 1 for pos, 1 for neg... Within each sheath is stranded copper.

I'm thinking of splitting the 2 insulated sheaths to a certain length, stripping the ends off of one of them to expose the stranded copper, and then spreading the strands to put between a split-apart sponge soaked in salt-water.

Would this work?

If you're planning to use the sponge itself as a switch the schematic might look like this (where "_" stands for a lead):

(needle) ---<(unit)]_______________(sponge switch)

If you plan to use e.g. a footswitch, the diagram might look like this:

(needle) ---<(unit)]_____/ (footswitch) __________(sponge)

In either case the lead to the switch would connect to the contact of the electrode which you would normally touch to complete the circuit. (That electrode should then really be disconnected to avoid completing the circuit inadvertently.)

If the sponge is the switch you would touch the sponge to complete the circuit. If you use a real switch you might strap the sponge to a safe spot and use the switch to complete the circuit.