I hope you will return your goldfish. 4 goldfish will need 50 gallons of water. I have 4 goldfish in 50 gallons and nitrates rise in 4 days. I'm doing a 50% water change every 4 days. Smaller tank will require more frequent water changes and stunt goldfish growth resulting in early death. Goldfish are little in the beginning but grow quickly. My 1 inch telescopic in 6 months has grown to 6 inches with tail.
Search jasmines goldfish tank journal. She has a great thread about goldfish and planted tank.
My experience with my goldfish....swords, annibus, and tiger Lilly go untouched. Crypts are a bit hit and miss. Wisteria, duckweed and water lettuce are destroyed and eaten.