As far as I understand it, strictly speaking, meat is allowed in Sikhi... as long as it is not killed the Muslim way (extrapolated to also mean kosher since the method is similar). Since Sikhi believes that the essence of the creator resides in everything in creation, the animals and even plants are no less a part of that creation than we are. So in that sense, There really is no distinction between plant and animal when it comes to being a part of Waheguru's creation. The reasoning behind not allowing meat killed the Muslim way comes from the fact that animals have nervous systems and a higher consciousness than a plant would, and killing that way causes unnecessary suffering. Also, it makes no sense to sacrifice something in the name of God, when it's God's very own creation you are sacrificing! Hence, Sikhs are against praying over an animal in this way as they are killed. So as per the SGPC mainstream rehet, meat is allowed in this context.
Having said the above, there are sects (eg. AKJ) that have more restrictive rehets that they follow. Among other things like no jewelry at all (when the mainstream rehet says only that ear / nose piercings are not allowed), no makeup at all (when the mainstream version says only that mehendi is bad), they also extrapolate the meat issue to mean vegetarianism. Then of course, there are varying degrees of that as well... some allow eggs and some don't for example. You would know if you took Amrit under one of these sects as Panj would tell you what is allowed and not. You would likely know ahead of time anyway, and it's your choice to take Amrit with them or not anyway.
Of note though, most Amritdharis I know, even under the main rehet, are vegetarian by choice. There are many spiritual paths that advocate spiritual progress being hindered by eating meat, since it's more primal basic and animal in nature... there are MANY people on many different spiritual paths (not just Sikhi) who say that being vegetarian definitely helps with their spiritual progress, meditation etc. by lifting them above the basic animal instincts.
And getting back to the OP...even Muhammad heeded the warning "Do not allow your stomachs to become graveyards!" and so many Sufis are also vegetarian.
