Nice 120 gallon, gotta love those oscars! I have one myself, his tankmate was sold to me as a "sweetheart bloodparrot" but is actually probably a midas x sweetheart parrot... she definitely has a lot of midas blood in her, whatever she is. She
is tailless though, and genetically so, thank goodness... no tail cropping

. Mine think they're a pair too, so funny. They never actually mate though, they can't agree on a location

. The midas hybrid laid eggs all by herself once, though, when they were still young and in a split tank.
Fire eels get around 3'... personally, I'd go with a bichir or two of some kind for that tank, they could better stand up to aggression should the cichlids decide they don't like them, and hey, bichirs are just awesome!
There are a lot of girls in the betta section, not sure how many are in the lower age range though... I'm a girl and 20, btw