Is it ever worth recruiting low level demons for fusion material, or should I stick to ones near my level?
Yeah, aside from evolution (which I havent experienced) it's nice to have em as fusion fodder. Sometimes I fuse demons before they learn all their skills, and if I have the space I'll pick up a low level demon since they earn exp in stock. If they're low enough they'll learn all their skills VERY fast, plus I've had a few really useful skill mutations... I had counter become mediarama extremely early which was great, and it happened with a low level demon I just grabbed again one time.