Goldfish tank...

[Flaringshutter]

To be rather honest, my opinion is that you can keep 1 fancy goldfish in a 10 gallon tank IF the tank is overfiltered by ALOT, if the fish is fed a varied diet, water changes are carried out at least once a week, and has excellent water parameters. If you have nothing else in the world to do, its very simple.

That might work except that even the smaller fancy varieties grow to over 10 inches. In a 10 gallon, they would be extremely cramped if they got to full size, but more likely the fish would end up stunted and would never reach full size or normal lifespan. Besides, they produce so much waste... You'd need 6-10x standard filtration for it to work even in theory.

Powellmacaque, I would say that you could keep four fancies in your 55. If you want singletail varieties, you're looking at a much larger tank, especially if you want to keep more than one. I have a 120 and I highly recommend it. The only consideration when looking into larger tanks is the width. Wider tanks are always better, since ST varieties grow so large. A pond would be the ideal situation.
As for tankmates, goldfish do best with other goldfish!

Temperature isn't such a concern - they're extremely adaptable, and most members here keep their goldfish in 70-80 degree tanks. If you're concerned the pond would get too hot, make it deeper and add some floating plants to provide shade. Hardware mesh cloth or fine wire mesh would keep out the snakes and such.

 

[Notophthalmus]

White cloud mountain minnows are good Asian tankmates for goldies. If you end up getting a large tank, you might also consider dojo loaches. If you get a gigantic tank or indoor pond, Myxocyprinus (AKA wimple, AKA Chinese banded shark) and koi become possibilities. There also a number of European and North American temperate species which are suitable tankmates for goldies.

 

[Kashta]

You mentioned wanting a "community tank" around the goldfish. When people say that, they often intend to mix coldwater goldfish with assorted tropicals.. and that won't work.

Tank size and filtration is a major consideration, as you've read. But for compatibility, the only fish I know that you can put with them are other (similar type) goldfish, dojo loaches, and white clouds. These fish all share the same water requirements and are well suited in temperament. Apple snails work well, too, if you'd like.

BTW, I stopped including Ryukins in the same tank if apple snails are there. The two I had before really went after the snail's feelers... so I had to separate those to protect my snails. It's that pointy mouth on a Ryukin... too easy for them to nip at something they see as a wiggly worm.