Scenario: Traditional Client management tools for Windows Device, no MDM.
Goal: To be able to use existing CMT, and set MDM to Intune.
Ask: 1. Do i have to configure Hybrid Azure AD joined and then use the GPO for Auto enrolment to Intune?
Ask 2: Does that GPO work on all win 10 Versions?
Ask 3: Is Hybrid Azure ad joined a necessity?
First of you can enroll devices manually, however I belive the best approach and also what you are looking for is automatic mdm enrollement of windows 10.
1, Yes to enroll devices into MDM you have to have Hybrid Azure AD join configured first and then you also need to configure auto enrollment both for Azure Hybrid AD join and Intune (preferably).
2, Yes it should work on any Win10 version, however you may see it differ fron a stability perspective. https://docs.microsoft.com/en-us/intune/windows-enroll
You also need Azure AD Premium (p1) or Intune for Education to get this to work.