The fish are not continually exposed to 80 degree or even 70 degree temps. Goldfish can survive with ice over the water..
That's great that he does keep large shoals of cories, the more the merrier, so long as the tank is not over stock.
As for the 55gal, that tank is nearly three times as much water volume as yours, but also provides a much larger footprint than your tank. As long as those aren't common plecos, a 55gal has room for a cory shoal or two, some small plecos, and a whiptail. Your tank is not that big. You could probably get away with a small cory shoal and a small pleco with some other fish, but not goldfish and angels. They're too large of fish for the tank at all.
You say your fish grow fine, your goldfish, if full grown, should be like the size of a softball. Are they? Or are they stunted? They may be newer additions so I'm not sure on that.. But if they're a year or two old and are in a well maintained tank that is larger enough for them to appropriately grow, they should be much larger than your fist. Angelfish can and easily will grow to be six inches long and eight inches tall. And they will reach that size in maybe 6-8 months, perhaps a bit longer but not much longer than that.
And temperature requirements have nothing to do with their 'preference', it's something vital to their survival. Rams will live much shorter lives in less than 80 degree water (really, more like 82). Fish like your cories will have a sped up metabolism which will cause an early demise, and this will be even greater so in your coldwater goldfish. They simply cannot maintain prolonged exposures to temperatures that are not even close to what is required for them to survive. There are typically wide ranges of temperatures fish can survive in, but you can't expect them to live long constantly exposed to it.
You shouldn't overstock a small tank based on plans to upgrade later unless you know for 100% certaintly, that you will be upgrading before these fish even reach anything close to their full grown potential. It's negligent. Many people on the fish forums are usually here because we genuinely care about fish keeping... People join to get opinions and do research on the appropriate care. If something sticks out as an issue based on scientific study, not opinion on the fish's 'preference', we will tell you. What I am telling you isn't my opinion. I will admit I do not and have not owned many of these fish myself. Because they are not compatible with my other fish, nor are they compatible with my source water.
+ Planted 75 Gallon Amazonian Tank +
Cardinal Tetras -- Rosy Tetras
Ember Tetras -- Silver Hatchetfish
Diptail Pencilfish -- Cories
Lyretail Checkerboard Cichlids