Guinea pigs can eat kiwi but as with any other fruit it should be fed very seldom in extremely small amounts as a treat due to the sugar content. The skin should be removed first and then cut a small piece the size of the end of your finger at any one time.
Kiwi fruits contain a high level of vitamin C which is essential for guinea pigs as their bodies can't produce this by themselves. However, sweet peppers are also high in vitamin C and a much better alternative that has more benefits to their overall health.
High in Oxalic Acid