Bruh, I speak Russian and seeing people call him Nikolai is so funny ngl.
It's like if you have someone named Ben or John and you call the Benjamin or Johnathan. Very respectful way to refer to your superiors and/or older people, especially with their patronym added.
As far as I know Niki (which would be an anglicized diminutive for Nikita usually btw) is of college age, so to me it sounds funny and/or weird. The usual diminutive for Nikolai is Kolya or Kolyan.
But that's a friendly way to refer to someone you know and likely is of age with you (within 10-15 years generally).