Given their size and their age, I doubt that they will need bigger housing for some time, and you could probably get away with a 55 long term, if that. For now, a good baseline to go for, would be 50% weekly water changes at least. Keeping a close eye on your water parameters will allow you to judge whether you can cut back, or need to scale up. Expect that as the fish get larger, and the tank gets more established, you will likely need to ramp up water changes to cope with the growing bioload. Be judicious in trimming your plants, as any nutrient uptake they provide will quickly be negated if they are allowed to break down in the tank. Try as hard as you can to get as much excess food and fecal matter out with every water change. Though it is often suggested that planted tanks not be gravel vacuumed, in this situation, where the fish are the main focus, it should be their needs you are more concerned with, vs. those of the plants. Aim to keep nitrate levels below 20 (better yet 15) ppm. The reason this is extra important in your case is because these fish have a lot going against them already. Given that the body morphology of fancy goldfish already presents some issues, compounding this with the deformation of internal structures often caused by stunting, means that these fish will be exceptionally prone to buoyancy issues. Pristine water quality, and a well balanced (goldfish specific) and varied diet high in roughage will work to reduce the the likelihood of these issues presenting themselves. I would recommend Mazuri's aquatic herbivore gel diet (5Z93) as a good staple, supplemented with high protein items such as blood worms and shrimp, as well as fresh veggies.
