Yeah, I've already thought of using mesh screen to separate them but the majority of frass are from the adults and they're larger than normal. Never really made a container that separates frass from small nymphs, I usually just dump everything in my large bin full of nymphs. I guess i'll just throw a few eggrates in there and if I'm lucky they will go onto it haha.
Also, I've been watching the adults and I found that some are behaving unusually. When I look around I always see a few roaches dragging their butts on the floor, almost as if they're scratching it. Could this be a parasite introduced by the flies?