IMO A snowflake eel and a Toby puffer would be fine in the same system although the snowflake may eventually outgrow a 75 gallon. Also they are both semi aggressive so you will have to pay close attention to what else you are adding into the system.
That will probably be fine. Snowflake eels aren't fish eaters in nature. I have seen them eat feeder goldfish, but this was more of a learned or desperate behavior...I believe.