It should be ok to put the air line in, the larger surface area created from the turbulence of the bubbles is what increases the amount of oxygen diffusion in to the water. You should make sure that you remove the air line before turning off the pump as a backflow of water will ruin it.
And for goldfish, the recommendation (similar to Betta enthusiasts recommending 1-2.5 gallons MINIMUM) is 30 gallons for the first fancy goldfish, and 15 gallons for each additional one. For common (comet) goldfish, the minimum is 55 gallons. Goldfish produce a lot more waste than other fish and can grow to large sizes, thus the large tank requirements. You can keep your goldie in a 10 gallon tank, but he will not thrive.