If you're in the US try craigslist, tell them the truth about them and give them away for free, hopefully someone takes pity and needs free fish, or see if someone you know has a pond. You can mix goldfish with certain 'tropical' fish as long as they can overlap in temp. Certain plecos, cories and others can go into a 'subtropical' setup but that's still usually towards the higher end for goldfish. When I did it I kept the tank room temp.
Anyways, I agree about finding oyt what kind of pleco too, because honestly if it's a common or similar I'd say rehome it too and focus on your other fish. That isn't a huge tank and it's easy to over stock :/
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