Veil tails are neat. Need 75 minimum, i would NEVER put one in a 55.
As mentioned, they are vulnerable compared to other oscars because of their long fins, making them great targets.
Also, oscars are extremely hard to sex. Venting is not always right, the only surefire way is have a pair and see which one lays the eggs (obviously that's the female), and then the other one (probably fertilizing the eggs...) is the male. lol