El Roberto wrote:

the goldfish they had were 7 or 8 inches long, much bigger than I expected
they'd ever grow.

2) Is that old story true about fish only growing as big as the tank you put
them in? I really don't want to end up with my 21 litre hexagonal tank full
of sea bass-sized critters....


Yes goldfish grow pretty big, and rapidly too. It's not aquarium size
per se that may stunt their growth but the poor water quality that you
get with growing/ large fish in small tank. They also eat plenty of
food -- produce fish poop etc that pollutes. Upshot is, that goldfish
need plenty of space even when you buy a little (young) one. Minimum 10
gallons per fish seems to be recommended. The water volume dilutes
pollutants, provides space to swim etc.

Your tank is 5 gallons, so 3 to 4 white clouds, guppies or neon tetras
may be suitable. The neons would need a heater...
d.