Can a Chinchilla and a Guinea Pig crossbreed?

no they can not. they need completely different food and should never be housed together for this reason alone. A chinchilla has a very sensitive digestive system. Not only is too many treats a bad thing but changing brands of food too quickly can cause huge problems for them. If you are going to change foods, always mix in the new with some of the old bit by bit until they are used to it. This is to not shock their system. Never give a chinchilla any kind of rabbit or guinea pig food. It can cause liver damage.