I think project plan should be created before to create the Stage plan. Creating an overall Project plan provides a great understanding of the entire scope of the project, which will then help us to build a great Stage plan.
Yes you are right. In the Starting Up a Project process, the stage plan is created for initiation as part of the request to the project or investment governance board to initiate a project. If approval is denied, then this represents the fail fast concept often referred to in agile.
If the project is approved, then the project manager creates the high level project plan as part of a product based planning approach that is part of the Initiating a Project process. Then as part of Managing a Stage Boundary process, the project manager updates the project plan and creates the plan for the next management stage (stage plan).
As per my understanding Project plan should be created first before Stage Plan.
However during Initiation Stage we are doing the project plan, and during starting up a project we are doing the Initiation Stage Plan, which is Initiation stage is a Stage. So the fist thing to do is creating Initiation Stage Plan which is a Stage Plan.