Yes , sikhism is monothestic . The concept is called Ek Onkar ( One Creater )
Opening linesof Guru Granth Sahib :
One Universal Creator God. The Name Is Truth. Creative Being Personified. No Fear. No Hatred. Image Of The Undying, Beyond Birth, Self-Existent.
There is but one God. Truth by name, the creator, all-pervading spirit, without fear, without enmity. Whose existence is unaffected by time, who does not take birth, self-existent, who is to be realised through his grace.
I, personally would list it under Pantheism, because there believe in the gods in much like the Hindus, they believe everything is god, as they contain the light of god and one day have to become a part of it.
But if you put them under monotheism then I would demand the hindus to be put under monotheism, because they do believe in one true god, Brahma. If you can't do that then shift Sikhism towards where the HInduism are listed. Thankyou.