55 gallons is fine for 2 comets, as we recommend water volume at 20 gallons per fish for pond varieties like comets, commons, shubunkins, wakins, and watonai (sometimes fantails as well).
I was a little concerned about stocking level here because in that video, the tank looks much smaller than a 55 gallon size. Have you confirmed the actual dimensions?
To reiterate what Lupin mentioned based on the details you've added........ yes, your comet appears to be quite stunted. My comet (sprocket) is already 10 inches long at 18 months of age and his mom (alexei) measures 12 inches long at approximately 3 years old. They continue to grow, however, throughout their whole lives... so it's still worth giving them adequate room. The stunting will cut down their natural lifespan, though.