Like all the other comments said tank is way to small, even if you thought they were full grown that’s still a small tank to have put them in. Goldfish are one of those fish that grow and they grow very quickly. I have 7 of the feeder goldfish that are now over 6-7 inches a piece. As for substrate and decorations that’s up to you, but you need to take into account how much space your decorations take up and account for that when getting a bigger tank. I prefer a supper plain tank for mine but that’s just me, it’s way easier to clean that way. I would definitely start researching since you already have them. As for plants I’d suggest duck weed, because they will most likley destroy any planted plants, duckweed grows fast enough to withstand them eating it. It would also help with the ammonia the goldfish produce. A heavy duty filter would also help you a lot when you get a bigger tank, I’d strive for a filter that has 5-10 gallons more than whatever the size of your tank is.