Wastewater is all run in the same pipes, they're not separated into grey and black unless there's some sort of water reclamation to recycle that grey water which would be unusual. Typically only wastewater and rainwater are separated.

I don't know of anything in the code requiring waste offsets in a high rise. On a vertical waste stack, the fluids still stick to the walls on the way down limiting maximum velocity.

If you're an architecture student, learn now that typically EVERY floor will have mechanical and plumbing. Some bigger stuff will be on those maintenance floors, but every floor between potentially has ductwork, terminal units, mechanical piping, waste piping, fire sprinkler piping, and water piping. Be kind to your engineers and leave some space in the ceilings.