joe how are you doing? i live in Arlington and two yrs ago built a 24x23 garage with Pex tubing in the floor for the radiant heat.....i have never hooked it up. my garage is 2x6 construction with 14' cielings roof trusses and well insulated . 3 windows, side door and large 18x8 garage door. the reason i never hooked up the pex was that i had an electric dayton 240v heater and for the time i use the garage weekends and nights i just turn the heat on and it warms it up to any temperature i want mostly 63-65*. it costs me about $1 a day to heat it. so with the high initial cost of the outlay for the radiant floor heating manifold etc i just cant bring my self to ante up the $$$ to pay for it. i had no problem laying the tubing and i knew before hand where the lift would go and at the same time i added more rebar to that area i moved the pex tubing away from it. i see no reason why you cant have more or less tubing in certain areas then others. for me while it would be nice to have i just dont see the cost justification for doing it at this point. running the tubing is the least expensive part of the project and is what i did so i have the abiltiy to add the heating source and manifold later if and when my current heating sources breaks.
there is no reason not to have a combination of two systems......forced hot air for overall shop and then the area where you have the machine shop or lift for working on cars have the radiant floor heating there.
you will want to insulate the under side of the slab so you have thermal break between the ground and what the slab you are heating.