Agree, you need a BIG tank or a pond to contemplate breeding goldfish. Any goldfish will breed with another, but if you want the fish to retain its breed characteristic, you need ot breed Oranda with Orandas, etc.
Even with relatively pure bred fish stock, you still need to cull the fish. Only maybe 10% will resemble the parents in fancies. With commons, you will have more luck. But as Crepe said, they need to be big before they breed, and your tank is not really big enough to let them grow big.