Welcome to Aquaria Central, Jooles!
I hope we aren't getting you all depressed about not being able to add more fish to your tank. The problem is that goldfish of all species can get pretty big. The shubunkin can reach 12 inches, and the other ones can get 6-10 inches long. I have seen fantail goldfish with bodies as fat around as 1-liter soda bottles. Most people will recommend 20-30 gallons for one fancy goldfish, and then 10-15g for each additional one. As far as your tank size, you're already doing good and you know more than some people who have been misled into thinking goldfish can like in bowls or tiny tanks.
A lot of stores and individuals keep plecos with their goldfish. The truth is that plecos should be in tropical conditions while goldfish do better at cooler temperatures. Also, as common plecos get larger, they stop eating algae and need to be fed fresh veggies, or at least a lot of algae wafers. This means that they make a lot of mess and don't really help to keep the tank clean. I am assuming this is why you have one, a lot of stores sell them for this reason. Also, common plecos can get quite large - 18 to 24 inches depending on exactly what species it is. Unfortunately, most people do not feed them properly and they die before reaching their adult size.
My opinion is to return the pleco, and possibly look into dojo loaches (weather loaches). They are a cool water fish that do well with goldfish. However, they do need to be in groups, so I would recommend that you get three of them IF and ONLY IF you are good with your tank maintenance. To add more fish to the tank, you will need extra filtration and a good water change schedule, maybe 50-60% once a week, or more as needed. If you are sure that you can keep the water clean and do the work for the tank, I think you could add the loaches.