If you just plunk a kitten down in the coop the Guineas will kill it. Just as with chickens an introduction needs to happen. Put the kitten in a separate pen or huge cage while the Guineas get used to it. You also need to think about the rambunctiousness of a kitten that may not play well with the Guineas.
BTW, the dogs need some training. If you bring a new animal on the property and they kill it they are out of control and need to be taught where their place is in the pack. I have three 100 pound plus dogs, I have cats that sleep with them but don't let a cat that doesn't belong enter the property or they're after it.