Forget about what sellers tell you re # of gallons - that's next to useless. Too many of them vastly overstate the capabilities of the stuff they are trying to sell Most people want to spend as little money as possible and the sellers want to sell you stuff - whether or not its ideal for your situation is not really important to them. Now there are sellers who will give good advice - look for ones who also professional pond installers/maintainers - their advice is less likely to result in fish deaths. Theres also another category of seller to avoid - people in garden centers who don't really know the answer to your question, and just read off the box and try to pretend they know what ttheyre talking about.

If you haven't already done so I would suggest doing the research to understand the nitrification cycle and various type of filtration systems out there (generally some combination of mechanical and biological filtration is necessary) and how pump size correlates to the filtration. A rule of thumb is that the water should turn over 1-2x per hour, but then you would also need to understand how the height of the tubing (aka head height) and other factors play into the capacity of the pump. Biofilter - s/b 10% of pond volume or be able to provide equivalent surface area for beneficial bacteria to live on - I have had GREAT success with a matala biosteps filter for my 600 gal pond.

That said... an overstocked situation is really something else entirely. Not sure why anyone would want to do that unless they are fish sellers - look into systems that remove/replace with fresh water because filtration alone may not be enough to handle large quantities of fish poop & the resulting ammonia. Generally, you should stock only 1 goldfish per 50 gals maximum - they will grow within 2-3 yrs. Generally if you stock appropriately you shouldn't have massive overcrowding because the fish will eat the eggs during spawning - I know it sounds kinda gross, but only until you realize its nature's birth control and the alternative is not good!

Goldfish need a lot of oxygen - an irrigator to pump O2 into water is essential especially if you have more fish then you should in that size of pond.