Have you thought about adding some easily removable type of structure in the pond so your YOY SMB can escape and hide from your brood fish?
From Brettski's pond project.
I need to fish harder for the SMB to remove the brood fish, and also and going to stock fewer brood fish and take my chances. I only need one pair to nest to provide all the YOY I need, so I'm thinking about limiting numbers to 8 confirmed sex SMB to ease in removal.
I'll post photos of my beds - the pallets, rocks, and PVC provide tons of places for the SMB YOY to escape - so I should be ok. Still, adding some removable structure is added security while the brood fish are still present. Good idea.