you shouldn't be letting any pet interact with rats cats dogs guinea pigs chinchillas. You never know when the rat or even the other pet can get hurt. Even if cats and dogs don't show any predetory interest because you cant train instinct out of an animal.
The main thing is that if you are to bring other species into your household, be sure that it is never an impulse purchase first of all, but also realize that so long as you supervise your animals, all should go well. At my house we have 5 dogs, 3 cats, 6 chickens, a turtle, and later today, 2 rats. Everything gets along fine, and half the time the chickens and my German Shepherd 7 month old pup will be napping out in the yard together. At other times he might be chasing them, or you see a hoard of chickens chasing a frightened pup. No matter how nice and calm they may act at times, you must always be aware that it is simply impossible to know what your pet might do next, so always pay close attention. I've never had a bad experiance though with the large mix of pets we have, they learn to live together.
As others have said, your rat should never have any interaction with your guinea pig. It is possible for guinea pigs to hurt your rat, but more likely your rat would injure your guinea pig. Even if they didnt injure it, guinea pigs are very much prey animals and get scared and stressed very easily. Just the presence of the rat would really stress them out.
Keep them in different rooms or in secure cages, be wary that rats can slip through larger bar spacing.