To get all 7 rally units, you would have to unlock the following:
1. National Policy

  • Tier 1: Liberalism > Philanthropy or Despotism > Monarchy
  • Tier 2: Capitalism > Mercenary System or Socialism > Party Leadership

2. VIP Level 7

How to access my National Policy?

- Tap the Command Center Facility > National Policy

How to check my VIP Level?

- Tap the Commander's Profile on the top left of your screen

How to increase my VIP Level?

- Use VIP Points item to increase the level of your VIP status

Refer to this link for the Item Location