Eh, I really don’t think it’s much of a stretch for a total rookie to be in Ren’s unit.

Ren is basically attached to that Star Destroyer, right? So I would assume he uses the best troops that are stationed on that ship. Surely his troops have suffered casualties- Finn was the next man up. Like Silverwook said, maybe he aced his training.

And what makes you think Ren’s troops are “elite”? (If there was dialogue that indicates this, I apologize, because I didn’t catch it. Lots to process in this movie). They look the same as all the stormtroopers in the rest of the movie. If there was an elite unit of stormtroopers (say, the equivalent of Seal Team 6 or something), wouldn’t they be the best equipped, maybe have different armor, weapons, etc?