I think i got this right, but I'd like to make a sanity check.
You are absolutely correct. When a "Whenever you cast" trigger is triggered, the ability will go on the stack above the spell that caused it, meaning it will be resolved first.
Here's the sequence of play.
- I cast Wake the Reflections. Populate does not require selecting a target, so I don't have to make any choices about it when the spell goes on the stack.
- Young Pyromancer's ability triggers ("Whenever you cast") and goes on the stack. This puts it above Wake the Reflections on the stack.
- (Both players pass, so the top of the stack resolves.) We resolve Young Pyromancer's ability, creating an Elemental token.
- (Both players pass, so the top of the stack resolves.) We resolve Wake the Reflections. I make a new token of any type I already have, including the 1/1 red Elemental I got a second ago.