You are incredibly over stocked. You should look into upgrading immediately. That tank is minimum for just one of those Oscar with nothing else in it! It's not even close to big enough for two O, not to mention way to small for a single bichir, or the chinese hi-fin. Do you have any idea how huge hi-fins get? Spite being slow growers, they can grow up to 3' long, not to mention they're cold water fish.
That gallon per fish size rule is 100% worthless and shouldn't ever be used for a basis when considering a tanks stock. Also, your math is completely off.
60 gallon tank - 120 inches of fish
If you use your equation of .5" per gallon it would be 30" inches of fish for 60. But it doesn't really matter that the math is wrong because this equation is completely worthless.