Reasoning for Soul Warden and Ajani's Pridemate interaction
It's pretty simple. Angel/Pridemate has to enter the battlefield to trigger Soul Warden. This means when the life-gain trigger resolves, the Angel/Pridemate will be able to see this and trigger themselves. So it's not actually a matter of specifically ordering the stack at all. It's actually just a chain of triggers.

As a list of steps (assumes no responses from anyone):


You have Soul Sister on the field
You cast Angel/Pridemate
All players pass priority
Angel/Pridemate resolves and enters the field
Sister's life-gain triggers because a creature entered your field
All players pass priority
Sister's trigger resolves and you gain 1 life
Angel/Pridemate's ability triggers because you gained life
All players pass priority
Angel/Pridemate gets a counter