I visited the rice terraces in Longsheng near Guilin in 2007. It is a beautiful place and well worth visiting.
There is no need for special clothing other than the usual good walking shoes and clothes suitable for the days weather. There are sturdy stone foot paths throughout the mountains and villages in the area. At no time will you need to worry about your shoes or clothes getting wet. See picture below for a view of people walking on the paths through the fields.
The walk up the mountain is quite steep and the trek will take several hours at least. It is possible to make long distance treks lasting 2 days with an overnight stay on the mountain. You need to have reasonable physical fitness to enjoy this. However, for a small fee, you can pay two porters to carry you up in a sedan chair.
In your picture above, you see the rice terraces in spring, full of water. Most of the year however, the fields are not water filled as they terraces dry and the rice matures in summer and autumn. I visited in late summer and the rice was a golden yellow colour.
As for the mosquitoes. They are no worse here than anywhere else in China. I personally don't wear any repellent and haven't had any real problem when outdoors in the six years I've lived in China. Mosquitoes usually bite me at night. Check your hotel room for mossies before going to sleep and keep the mosquito net on the window closed.