Nexus questions

do you think it's a good idea, to reduce dependencies, plan one Scrum Team's Sprint at a time before moving on the next team?

It is a good idea to reduce (and eliminate, where possible) dependencies. However, planning each Scrum Team's Sprint one at a time is not how Nexus accomplishes it. Dependencies are identified and reduced through the act of Product Backlog Refinement. A simple act of changing how the work is decomposed into Product Backlog Items can help to manage dependencies.

Nexus Sprint Planning is defined as a timebox that starts with a Nexus Sprint Planning session followed by the teams breaking off into their individual team Sprint Planning sessions. There's nothing that says that all teams must conduct their planning sessions at the exact same time, but I don't think that enforcing rules on your Sprint Planning sessions to make up for problems in refinement is in the spirit of Scrum or Nexus.

and may the NIT select work for the Scrum Teams in very complex situation?

I don't think that this is in the spirit of Scrum, with self-organizing and self-managing teams. Scrum, and therefore Nexus, is designed as a pull system, where teams pull work into their Sprints and then execute on that work. Having an outside entity select work and direct teams to start it is the opposite of what is desired in Scrum. Therefore, I'd suggest that a situation where the Nexus Integration Team is pushing work onto Scrum Teams in the Nexus should be avoided.

• do you think it's a good idea, to reduce dependencies, plan one Scrum Team's Sprint at a time before moving on the next team?

Is that even possible, considering that Sprints run back-to-back and with no gaps between them?

• and may the NIT select work for the Scrum Teams in very complex situation?

How would those teams then be self-managing, and their skills fully leveraged, in this complex situation?

• do you think it's a good idea, to reduce dependencies, plan one Scrum Team's Sprint at a time before moving on the next team?

No. Dependencies should already be identified and reduced prior to the individual Scrum team planning.

• and may the NIT select work for the Scrum Teams in very complex situation?

No. This will break Scrum since the Nexus framework is also Scrum. The NIT, however, should coach the Scrum Teams on how to resolve integration issues. 

Thank you for your answers and support!