I have 5 cats and many guinea pigs. There would have been no way I would have let any of them near my pigs until they had grown up as they are so unpredictable as kittens. They are now grown and I know each and every ones personality, likes and dislikes. I trust them all with my adult guinea pigs, HOWEVER, I will never trust them around guinea pig pups! If they see a small baby running, scurrying, and just plain old popcorning, as they tend to do, my cats go into hunt mode at the drop of a hat.
All of my cages are cat proof, the only time the cats get near them is during lap and floor time and then it is only the adults that I let them near. I still watch the interactions, and most of the time my cats won't even bother them, they have no interest in them. They turn their noses up so to say "oh, it's just those things".