I'm sorry Live Fish Guy ? TMRC is perfectly correct in everything he is telling you. I spoke before realizing what size tank you are moving up to (Sorry TMRC). Goldfish, produce immense amounts of waste as does that Pleco (I am assuming the most commonly available at a pet store). Please refer to JLK'S sticky on size reference for goldfish.
I cannot possibly explain to you how fast and large goldfish grow. I have a 120 gallon with 4 Fancys. My Dash is a Red Cap Oranda/Ryunkin that I purchased about a year ago. She is already humongous. Most likely there should only be two Goldfish in there. I have a Fluval FX6 Cannister on this tank. It has three trays filled with biomedia, foam around the outside of each tray and a course and fine media pad on two stages. If I rember right, it is rated for 300 gallons. The tank is well-seasoned, 4-5 years and yet I do a 75% water change every week and clean the filter with tank water every 3 Weeks.
That 20 gallon should only have 1 goldfish in it but you could definitely get away with two small if you realize that you will be upgrading again soon.
Please find a home for the Pleco. The most commonly found Pleco at pet stores grows 2' +. There are varieties that stay much, much smaller like the Rubber or Bristlenose, 4"-6".
Please do not add any more fish.
I don't wan't to hurt your feelings, so I am very sorry
I think the best way to help you understand your problem is to take a picture of my 120 gallon so you can compare that extremely large tank to the size of the fish. My saving grace is that my Lionhead and Lionchu are much smaller than the other two and grow much more slowly. I have had them longer and yet they are still smaller. My LFS can't seem to carry pure bred fish. I will not use him anymore.
Hang in there Live Fish Guy. Please don't be offended by TMRC'S advice. He is a proffessional and only trying to help you and your fish.
120 gallon picture to be posted soon.