Ben getting nerfed is understandable, because the entire episode was in Rex's world, making him the main character. You don't want to detract from Rex in that setup, because then he just looks bad from then on. If they make a reciprocal movie, Rex will most likely get nerfed/given a supporting role.
I kinda wanted to see Ben get pwned by Sixx. Going up against a trained professional who specializes in dealing with a wide variety of unusual creature could be an interesting setup.