Wake the Reflections and Young Pyromancer
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.