I use oats as substrate myself. It works as such and they eat it.
You should seperate them, my worms will chomp on the metamorphs just like the beetles.

You can sift through each week or cut the bottom out of a beetle tub and nut and bolt a screen on the bottom.
Put that tub into another tub with oats as well so the fresh hatched babehs wiggle through the screen and you wont have to dump and sift every week or two.