How to cook rice for 60 people
Three main factors you need to think about is the water ratio, method of cooking and portion size.


When cooking Japanese rice(short glutinous grains) the optimal ratio is about 1:1 of water and grain. Normally when cooking smaller portions you add additional water to compensate for the water that evaporates and turns to steam while you cook so in most recipes you will see recipes asking for 1 cup and a couple extra tbsp of water for each cup of rice.

However you are cooking a very large numbers of servings so the evaporation will be negligible. You can stick with 1:1 ratio.
To cook the rice you can use either a large stock pot on the stove top or put it into a large pan and bake in the oven. 

For the first method, you need a really thick stock pot(a thicker pot results in better heat distribution which results in less burnt bits at the bottom). I would recommend doing 30 servings at a time. Measure out the rice and water and bring to a boil. Then turn the flames down really low and simmer with the lid on for 30-45 minutes. Then turn off the heat and let it sit with the lid on for at least 15 minutes.

To make it in the oven, measure 30 servings of rice and water in a large oven safe pan and bake at 325F for around 30-45 minutes. Turn the oven light on and check if all the water have been absorbed into the rice. Using this method I would increase the amount of water to about 1:1.1 of rice to water.
Rice will generally double in volume when cooked compared to its uncooked version. If you are making 60 1/2cup servings of rice you would need 15 cups of uncooked rice. Or if you are going with 3/4 cups per person, you would need 22.5 cups of uncooked rice. 6 kilo of uncooked rice is around 25 cups so your estimation was pretty close to the mark. I would cook all 6 kilo of rice. Its better to have a bit extra than to not have enough.