I have had 2 large snow flakes and they never bothered anything. Even had small fish in there they could have eaten. However, I do want to mention that I feed well. If you do not keep an eel fed well, it will hunt. The only problem with a snowflake is that they escape out of the tank really easy. You need to lid or it will end up on the floor.
Right now, I have a jeweled eel that could easily kill some of my smaller fish, but its fed well and leaves things alone including my cleaners.