That is where a lot of ours are. We tore out the old sheet rock, put up insulation and new sheet rock on the walls and the same for the ceiling. We installed a window unit a/c and modified our gas central heat and air to heat the garage as well. We sealed the concrete floor with drylock to keep moisture down. We also used cheap peel and stick tiles to make it look less like a garage.
We sealed up any gaps around the garage door, which is still functional but locked. It comes in handy during the spring and summer months. I can open the door and clean rat and snake cages and the water hose is around the corner and trashcan is at the curb. During the winter, we have to carry bags of dirty bedding and papers through the house the long way around.
I love having them in the garage. Much more escape-"friendly" than the house, no kids, no dogs, no cats can get out there and torment the animals....
If done properly, yes you can keep them in the garage. I run a humidifier and with the window unit running, keeps the room temp about 80 degrees, or if I know I'll be cleaning cages for a few hours, I'll turn the a/c up and cool the room off a bit.
~Beth
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Some people are like slinkies, Completely useless, but bring a smile to your face when pushed down a flight of stairs. 
