Leave the Project Manager group with 'Create' rights but then create a record rule for project managers that restricts create permissions with [('create_uid', '=', 'Administrator')].  This way, Project managers still get to add pre-existing stages to their project from the project stages tab in the form view, and they can try and create a new rule from within that 'Add: Task Stages' screen, but they will be met with an access error when they click save, unless they also happen to be the Administrator.  This is what I was aiming for.