i agree about the tang, but since you have only a 75 gallon, a yellow tang is probably all you will be able to hold. Or another type, but only for a little while as they will outgrow the tank and need more swimming space. I dont think it matters as far as the snowflake eel or the lion. Most snowflake eels, as long as you get them small, are very docile and peaceful. i know mine is. He did gat mad though when i added my valentini puffer, but only when he tries to steal his kirll of the feeding stick. Lions are also peaceful fish. They are only classified as agressive because they will eat anything that can fit in their mouths.
what type of lion are you looking at? A V.lion will outgrow yout tank. But you could go with any of the other species, and you have quite a selection with sizes ranging from 4-10 inches.