If you want happy and healthy reptiles, consistency is important, as is avoiding extremes in either. I imagine it would be difficult to maintain consistent temps and humidity, even with proper thermostats and good caging (insulated - like Boaphile, AP, etc), if you are opening the garage door at all. I also think opening the garage door would lead to either spikes or drops in humidity, or in FL, often keep the humidity too high for some species. Both Carpets (Bredli) as well was BP's, are prone to RI's if not given good and ideal conditions. Temp changes and humidity changes can cause these, among other illnesses.
I keep my reptiles in an area where temps are very consistent as is humidity and there is no drafts. I also use top notch caging and thermostats.
If you can insulate, not open the door, and/or build a small area in the garage that is insulated, I think it would be okay. However, probably not a good idea otherwise.