Chinchillas are vegetarians, primarily grass and seed eaters. They will eat alfalfa or hay. I find that alfalfa is the best as it has more leaf and less stem, and is richer than Timothy hay which is a favorite of breeders. Feed fresh pellets, and if the only really fresh is guinea pig or rabbit, then that is ok. Some people will tell you that rabbit pellets have hormones in them and are bad for chinchillas. That is a myth, and the reverse is true.
They generally love dried fruit like raisins, cranberries, etc. – but should be given these in moderation. Some also love fresh vegetables like steamed corn on the cob. Each one has specific likes and dislikes, so offer small tidbits to see what they like. Be careful however, they have a tender digestive system and can get diarrhea from too many treats.
Give them a little salt block and a few pressed alfalfa blocks. Since their teeth keep growing, they need lots of good gnawing material. A pumice stone is great, if you can find one. Various woods are also good, such as pine, apple and manzanita. Avoid woods like fir, spruce, plum or redwood. Give some other supplements which you will find under health/supplements. Treat the chewable vitamins as daily treats, and they will eat Vitamin C and enjoy it as much as they do other sweets. You can give each of you types of pellets separately.
However, whatever you give should be fresh and not stale or spoiled. Smell your pellets periodically to be sure they are still fresh.