You really either have to take your goldfish back or get a bigger tank. Plecos really need a 55g, and even that will be cramped when its full grown(until then you will stunt it and possibly disable it, resulting in a shortened, sickly life), 75g better. Gold fish need 10g a peice and need cooler water than the cories and pleco. BTW-I have often heard that (also) plecos can develop a taste for the goldfishes slime coat, ending in usually death. You would be benefited by feeding your goldies and pleco veggies. Zucchini, squash, peas, cucumbers, all blanched or whatever. Goldfish need salt, which may or may not agree with your other fish. None of this will matter, if you don't get a bigger tank. They will die. Most goldfish live well above 10years, closer to 20yrs to my understanding, six months is no time to them. You will have nothing but problems in the future, I promise. I would go back to the same pet store and complain to the management, they should accept the return and I would be very unhappy if they did not at least give me something more suitable for a 10g, if not my money back. Unless you get a 55g I would stay away from the common plecos. Cories love their own company, if you got rid of the pleco and goldfish, you could get a couple more cories and a Oto or two being better, I would at least get one more cory, after you fix the current problem.
If you have anchor worms, I have heard you can manually remove them. Are you sure it was not fuzzy? Could be flex? Kind of cotton fuzz.