There are a couple of things you could do. You could move everything from the 10 to the 28, fish, existing AC20, and decorations, and let the new AC50 run along side the old for a few weeks. At this point remove the old smaller AC20, keep the media wet, and put it back on the 10 gallon when you are ready for fish. Before you do this, don't feed the fish for 20 hours, and feed lightly for the next week, about 1/3 normal. This will prevent any spikes.
If you are itching to get the 10 gallon up & running you could use half the sponge, again, the same fasting & feeding routine would apply. Remember to keep the media wet, if it dries out you will lose nitrifying bacteria.
Goldfish are actually cold water pond fish, get quite large and are very messy. A 10 gallon is way to small you need 30 gallons for the first fish, and 10 gallons for every additional fish beyond that. Platys look similar to goldfish, don't get anywhere near as large, and come in a large variety of colors. A fish such as this would be more suited for a 10 gallon tank.