For anyone who doesn't know: Sikhism is a monotheist religion. It is "unitarian", of course, so anyone who holds to trinitarian ideas will not necessarily find it completely compatible with Christianity.
It is not Hindu, despite a few similarities. Not everyone who is a hindu is polytheist, anyway.
Seconding this. It is worth noting that in the United States, Hindus, Sikhs, and Jains occasionally share holy spaces out of necessity because of the partial overlap that does exist, but they are three ultimately distinct traditions.
I don't know if even
most Hindus are "polytheistic." In a literal sense, most Hindus revere more than one deity, of course, which is a form of polytheism, but it really would seem to depend as much on philosophy as individual psychology. Are deities "Gods" or are they godly? I'm not sure there is a consensus within Hinduism.
Finally providing an actual take on the topic at hand -- I am not as well-versed in Sikhism as I am in other Dharmic traditions. I was trained to view Sikhism as a synthesis of Hinduism and Islam, and I've come to disagree with that view on a historical basis, though there is a useful Venn diagram to be made there.
I appreciate much about the relatively basic information I have retained about Sikhism. The main reason I have not investigated Sikhism more on a personal level is that I do not find it especially helpful to demarcate the godhead as an entity in and of itself separate from Being, or as singular in nature. I prefer a totalistic characterization, and I have found the Upanishads personally resonant to that end.
I also am not very keen on the level of reverence Sikhism seems to require for specific authors who wrote specific things and made specific contributions to "Sikhism-in-and-of-Itself." I recognize this is not inherently any better or worse than using reformers to justify abstract doctrinal/thought movement, or using figures of debatable historicity to communicate the same ideas. I just find it harder to accept.