Kumquats are fruit that look quite like oranges but are smaller and more oval in shape. They are native to south asia and the asia pacific region and are eaten raw. They have a sweet rind and a sour centre.
Kumquats are usually eaten whole in order to get the contrast in taste of just the rind is eaten.
image flickr darya pino

So if we can eat them, can guinea pigs eat kumquats and if they can how much can they have?
Lets take a look by taking a look at the nutritional data of kumquats in more detail.
As per usual we’re looking closely at its calcium, sugar, oxelate acid, fat, and phosphorus content as far as guinea pigs are concerned.
Energy 296 kJ (71 kcal)
Carbohydrates 15.9 g
– Sugars 9.36 g
– Dietary fiber 6.5 g
Fat 0.86 g
Protein 1.88 g
Vitamin A equiv. 15 μg (2%)
– lutein and zeaxanthin 129 μg
Thiamine (vit. B1) 0.037 mg (3%)
Riboflavin (vit. B2) 0.09 mg (8%)
Niacin (vit. B3) 0.429 mg (3%)
Pantothenic acid (B5) 0.208 mg (4%)
Vitamin B6 0.036 mg (3%)
Folate (vit. B9) 17 μg (4%)
Choline 8.4 mg (2%)
Vitamin C 43.9 mg (53%)
Vitamin E 0.15 mg (1%)
Calcium 62 mg (6%)
Iron 0.86 mg (7%)
Magnesium 20 mg (6%)
Manganese 0.135 mg (6%)
Phosphorus 19 mg (3%)
Potassium 186 mg (4%)
Sodium 10 mg (1%)
Zinc 0.17 mg (2%)
(source: Wikipedia)
So can guinea pigs eat kumquats? Taking a look at its nutritional data, it contains a little phosphorus, the amount of calcium is a concern, and are very sugary. They are also very acidic.
However, they do contain a terrific amount of vitamin c, a whopping 53%!
Because this they only worth feeding to guinea pigs occasionally. Maybe once a month at best and I would only feed one of them to your guinea pigs. However, the vitamin c level makes it worth feeding it to them, if only to add some variety to your guinea pigs diet.