I think that would depend on the type of eel. Some are much more dangerous than others. I have two percs and a golden dwarf moray together with no problem, but people have shared experiences to the contrary. All eels are predatory and it's a risk.
I was just given a 240g with a Zebra Moray and a Snowflake eel AND a Tomato Clown. The clown is definitely an adult but the eels don't bother it at all. They are rather sedate but then they are old too.