The baking soda is a catalyst to aid the reaction. You can use Arm and Hammer washing soda, and it is actually recommended. As to what is happening is not that the rust is being removed, but rather than the metal is actually being un-oxidized, that is, it is turning back to iron. The more re-bar electrodes the better because the reaction is more or less line of sight, that is, if a straight line from the rebar would not be hitting a face of the rusty metal, it won't react very well if at all. In other words, the more electrodes the better. I personally would suggest sticking with re-bar as it will degrade over time and it is cheap. Also the one article mentions that using stainless will render the water/washing soda mix toxic and is supposedly illegal.
