Well I did watch a video of some guy redirecting a class E. He only used a single somewhat small craft. It did take a lot of time and delta V, but he redirected it regardless. I just think that compared to the size of real asteroids, a class E is pathetically small. I know that there are limitations, but I think it could be possible to redirect one twice the size, or a little under twice the size, using multiple craft. Obviously it would be borderline impossible to redirect an asteroid any bigger than about 35 meters, which is exactly why class F's would have to be significantly rare, and would be no bigger than 30 meters.