Dean,
Regarding the use of a trowel machine. A trowel machine is the only way to get it superflat and finished with any degree of consistency. Finishing a large garage floor by hand is just wrongheaded. I was in that business for 20 years and did nothing but warehouses, laboratories, research labs, manufaturing plants and other commercial/industrial concete floors that were held to extremely strict tolerances. The only thing we ever finished by hand generally was the edges and around imbedded items and columns. It sounds as though your contractor knows what he is doing, just make sure he puts a finish on the surface that the is in accordance with the stain manufacturers' recommendations. A reasonable tolerance would be 1/8" measured in any direction with an 8' straitedge. Hope that helps.
Regards,