If We Decentralized Everything, Do We Get Communism?

No, not really. Maybe. Kinda-sorta.

The communism of the dreamers who want utopia is actually gotten to through extreme capitalism. (Whereas the communism of central-govern-cement is we're in charge of everything)

Any organization that is too large will fail the majority of people.
I mean, look at the DMV, they can't even do their stated job. And what they do do is slow and twenty years out of date.

So, to answer your question, decentralization is not the correct term.
The more apropos term would be power to the people.

As in, we should be building small community sized green/renewable electric plants everywhere.

Individual electric plants are too complex for the non-mechanically minded to take care of. So, really you need a specialist to keep it running. Thus the minimum size is actually about 100 people.

The global electric grid with one power producer called something like ENwRONg doesn't do anything correct for the users. It has no incentive to, and can't move fast enough for any change.

So, to answer your question, there is a level of decentralization necessary, but there is also a level of cohesion necessary.

And Elon Musk isn't the genius his marketing has labelled him as.