There's no gold fish that can live in a tank that size. They all get at least 6 inches most of them more. I wouldn't put a gold fish in a tank less than 20.
The tank is going to be in a really obsessively climate controlled room so the room temp will be constant and the tank's internal climate and filtration controls shouldn't have to deal with peeks and valleys.
I'm thinking of maybe some micro world, puddle life sort of scale. I know there are little creatures that can live in 5 gallons or even less sometimes. Maybe I'll make a local fauna tank ... I'm still open to suggestions for fish that are known to be interesting to watch.