If outside temps are suitable for the species, I would just concentrate on moving enough air to keep temps reasonable. A good ventilation fan would keep temps much closer to outside temps, and would be much more economical than AC. Insulation always helps provided you have a way of removing the heat that does build up. If your collection doesn't require a whole lot of space, constructing a small insulated "room" within the garage, might be the way to go. Then if need be, you could AC just that small space.
I keep my rats in the garage. My main problem isn't heat, but rather the cold. I just insulated the outside wall right behind the rack, then made some walls and ceiling out of plywood and lined it with that foam/foil board insulation. It's just big enough to surround the rack and a small oil filled radiant type heater. Heater kicks in only when the temp falls below 45 F. Keeps the watering system from freezing, and the rats breeding.