Can my cats survive in garage? should they be outside cats?

As long as they have warmth, food, and water, your cats could survive in a garage, but giving them access to the outdoors would be much better. Even though they are domesticated [kept as pets] they have natural instincts. They can fend for themselves in the "wild". There are dangers: cars, dogs for example, but in most cases your cats will be fine. As long as they have claws to climb and catch birds/lizards, they will be happy and relatively safe. If they are afraid to go outside, as one of my cats is, then they will be ok in the garage, just be sure to let them get out every now and then to the rest of the house or something for exercise. Hope this helps!