New Neurons for the Brain

That seems to be one of the findings of the paper- the new neutrons were self-organising, in that they could find the right kind of other neutrons to attach to. How far that self-organisation would extend is an open question however.

Well, what about smooshing together three human brains? From my basic knowledge of neuroplasticity, if the brain area is unused the nearby areas will take over and use it for their functionality (e.g. blind person's increased sense of hearing and smell)

From the article:

"Now researchers at the Max Planck Institute of Neurobiology in Martinsried, the Ludwig Maximilians University Munich, and the Helmholtz Zentrum München have demonstrated that, in mice, transplanted embryonic nerve cells can indeed be incorporated into an existing network and correctly carry out the tasks of damaged cells originally found in that region."

Is there any findings / publications on this?

I haven't seen any evidence to support the claim that psychedelics other than Ayahuasca catalyze neurogenesis. That being said, there is evidence that LSD increases neuroplasticity. https://www.cell.com/cell-reports/pdf/S2211-1247(18)30755-1....