TWO ORANDAS ARE PERFECT there. But nothing else. Goldfish kind of clean the bottom themselves
But know this, no fish, no matter if it's a pleco or not would eat sludge and waste. So you still need to clean the gravel well during the weekly waterchanges
Bel: 20gallon for one fish means the measurements of the tank! Not the volume. 10 gallons is just enough for one goldfish, but a ten gallon tank would be too small for the fish to live comfortably.