Although submerging a rabbit in water is never a good idea, since they can go into shock, gently misting them with a spray bottle is okay. Some people use squirt bottles to train rabbits to not to do things, and, although I don't agree with doing that, spraying him to cool him off would make him too confused, so you might have to scratch that! Frozen water bottles can be refrozen again and again, so one or two by your rabbit's pen area is a great idea as well. Some owners get desk fans and put them by their rabbits, which is another great idea. Put some ground-up ice in their water bottles or bowls to keep your rabbit refreshed and cool.
If your ferret seem drowsy and hot, wring out a wet washcloth and put it in his cage or leave it dripping and hang it over the top so it drips down. Fans work great for ferrets, and ice in water containers keep them very cool. Ferret waste gets smellier when hot, so be sure to clean it out so as not to irritate your ferret's senses too much.
Your guinea pig will also need a fan on low speed. They have stressed systems, and sudden cold like an ice pack or blast of cool air will cause them to get very stressed, and eventually die. Put some ice in their water as well, but don't push too much cold.
Never, ever, ever, ever, ever, bathe your hamster. They have special oils in their skin that are scent marks for other animals and hamsters. If you have Russian dwarfs together, they will not recognize the scent of their hamster friend and reject him. Also, chubby hamster bodies cannot tread water or swim!
A small fan will also help those little guys well, but really, they can keep cool well enough without help. In my experience, my fancy Rex-furred black hamster slept on the bare cage bottom because it was cooler, and as long as the heat indoors isn't too hot for you, he should be fine.
Rats and Mice:
Bath rats? Yes! Bathe mice? Nope.
Rats are natural swimmers, but mice won't be able to swim. Use the hamster keep-cool remedy for a mouse, but if you do want to give your rat a bath, go for it! Actually, a kiddie pool with 5-inch deep cool water would be fine, as you'd only bathe a rat if he needed it. Rats love to play in water, and put a few dog kibbles or cucumbers or peas in the water to float for a little treat. Your ratties will all love the "beach party!" Be sure to dry them off after the water playtime, and they'll be happy. If you don't want to break out the pool, then a fan or iced water bottle will be fine. Rats love to roll the iced water bottle around and it helps them to stay cool. With rats, water is completely harmless.