when I first got mine, at 3-4inches, she wasn't aggressive either...till one day months later she turned on my much bigger oscar. The oscar survived, but was in ICU for weeks (insides turned out)
She was fine as long she didnt' feel threatened...until these last couple weeks...i"ve had her for over 1 1/2 yrs with the same catfish...she has turned on him...I'm actually using the mirror trick to "scare" herself...she will turn from it, or try to attack herself. Puts less strain on the catfish being attacked..
as for female, does the fh have "flower" markings on her top dorsal fin? If so, it's a female