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