Hehehehe...that was my old thread!
Anyways, here is my experience since then; My fire eel hides pretty much all day and comes out at night, just about the time the gars settle down after getting fed. Initially I was a bit worried as the eel was a couple of inches smaller than the gars, but Ive saw no signs of aggressions either way. If I could speculate which would cause problems to whom, it would be the gars to the eel.
From what I read on MFK EELS Forum, Fire eels are very peaceful except from other spiny eels.
I wouldn't worry too much about it, as long as there isn't a HUGE GAP in size. Also, if you provide a hiding space, which they very much like, I am assuming it would be even more difficult for the gars to establish aggression, as the eel will rarely be seen.
Be warned though, as Fire Eels GET HUGE...FAST!
After 2 months, my 6" fire eel outgrew my 8-9" gar by 2".
Note: Not my tank...just trying to show size of eel, not tank compatibility
http://www.youtube.com/watch?v=2jDD4NIS0TY