Siamese cats are domesticated animals and are typically fed a diet of commercial cat food. They are not naturally inclined to hunt and kill guinea pigs, which are much larger than their usual prey.
However, like any cat, a Siamese cat might be curious about a guinea pig and might try to swat at it or play with it. This can be dangerous for the guinea pig, so it's important to keep cats and guinea pigs separate to ensure the safety of both animals.