the atheist reality is not one cogent thing,
I believe that atheism is most likely materialism in a nutshell. Anything outside materialism falls within the categories of spirituality/religion/theism. I'm not going to sit here an argue endlessly over semantics, but that is the basics of what we've been dealing with in concerns with atheism for decades now. To try and change that now is somewhat redundant.
atheists can believe in spirits and ghosts and metaphysical realms
Lol. And now you have to account for transcendent realms and souls that have originated from somewhere (without God), and somewhere independent of how we understand the physical universe from a materialists perspective.
I as do not believe others to exists
That's what makes you an Atheist. Rather than a Theist or some label that is not restricted to materialism. Wow, you say one thing and admit another...
but atheism is not a group thing with regards to assumptions.
I've never met an atheist that supports metaphysical realms, spirits and ghosts. Even if there are Buddhist factions that claim they are atheists, they still have to account for what I said above. In other words there is no such thing, an atheist cannot account for metaphysical realms with an atheist worldview, it's nonsensical. I'd argue that with any person claiming they are atheist. And of course they would lose, as it only makes sense under a Theistic worldview. Because it only follows that God could be in control of multiple realities existing and millions of existing souls or more existing otherwise you get silly. Only one source can be necessary in terms of logic, that source being intelligent.
You can believe others to exist all you like, until you prove it true, I will sit here in reality.
So goes the monotonous battle between materialism and creationism (atheism vs theism). To call it anything else is a pipe dream.
atheists are simply people who lack a belief in god, they can believe any spectrum of things outside of that.
If you don't agree that metaphysical realms, spirits and ghosts are not proposed as religious concepts then you are just lost. They are specific to religious propositions and have been for ages. The soul, transcendent realms and spirits are fully articulated and understood through religious sources and spirituality only, to claim they are atheistic concepts is a denial of reality. Any atheist claiming theistic concepts would have to come and support such notions and how they formulate their beliefs.