To me i think it is more about how you feeding. Saying you feed you goldfish in small amount at about 2-3 mins eating time. And using filter that design for 600L. I think the filter will work for you. Additional filter will be need once you do less water change (once a week). However, if you do water change daily at 20-50%. I think you should be alright there.
Please note, more filter always work better for you especially when you do less water change and more feeding.
