Oh wow, an 80 gallon would make a stunning goldfish display tank! I hope @Deanasue is willing to share some of her large goldfish display tanks with you
The storekeeper gave one piece of accurate info - that goldies do need a friend, so happy you got two, you'll be able to have a few more with an 80 gallon, as long as you account for how big the common pleco will get as part of the equation of course. So glad you found out what they need, and were willing to do what it took to give them what they need - lucky fish! And an 80 gallon would be a gorgeous tank, I'm jealous! lol. You could make it into a real showstopper.
Glad you know about how big common plecos get, and have plans for what do with him, I just had to give you a heads up because far too many common plecos are sold to people who have no idea, then the fish ends up stuck in too small a tank and gets stunted, and they outgrow those small tanks really quickly, and people struggle to find a home for them once they're giant. If you can upgrade to an 80 gallon then eventually move him to a pond, then that's ideal.
Please don't add any more plecos to the 20 gallon though. You're really fully stocked with the two goldies and the pleco as is. Male plecos also get territorial, a male pleco can kill another quite easily.
(internet pic below). Once I get my new tank set up long enough I'm getting an Albino or Lemon Bristlenose 
