I will be putting down reclaimed flooring over hydroponic heating coils. I will glue and screw 2 x 3/4 solid fir furring down 8" OC on the old plywood underfloor (on top of reflective insulation). I want to lay the flooring with say "a quarter" as spacing between each row, to allow for expansion/contraction. I am thinking that I need to make sure the water content of the furring is below 10%. Will this work? What type of connector should I use? nails? staples?

A quarter? I don't even do that in gymnasiums. I doubt anyone would rent something as sensitive as a meter. You can buy a mini ligno for around 100 dollars and probably sell it on ebay.