once the fire eel outsizes the peacock I'de have aggression concerns. they may be fine. or the fire eel may kill the peacock before the other fish grow big enough to eat it. I have a smaller TT eel I wouldn't dream of putting in with my 3 fire eels. They are a rough and tumble trio, and quiet aggressive towards each other when it suits them. and TT's are well know to be aggressive as well. sexual maturity will spell aggression between the eels in all likelyhood.
It has been done, but I personally wouldn't do it.