I'm a kitten foster. I foster underage kittens and keep them until they're old enough to get fixed and adopted. It depends on the animal... I know puppies are a lot more work!! Kittens are actually not too hard to take care of if they're already 4+ weeks. If you already have guinea pigs, or you know how to take care of them, I don't think it would be a problem.

Bad things about fostering:
There is no bad! The only thing is that it CAN be hard to let go of them. I ended up keeping my all of my very first fosters!! (Ilio and Lali)
Sometimes it can also get a little expensive since you have to buy them food and stuff... (the rescue I foster for pays the vet bills though)

Good luck!!