Who are the Nihang Singhs and what is their history?

I am unable to find much reference to Nihang Singhs.
A brief answer to your question is:
Nihang Singhs are an army of Khalsa. They are well trained in the use of the arms. Even now they are an independent army and have their own dress code and rules.
They seem to have come into existence during the time of Guru Gobind Singh Ji or soon after. Nahang Singhs were active during the times of Banda Singh Bahadur. There were two groups of Nhang Singhs, a group of young fighters, TARUNA DAL, and group of older fighters, BUDHA DAL, mainly in the support role.
One can find a lot more information about Nihang Singhs on the Internet.