Can you put guppies in a 1 gallon tank?

One goldfish needs 20 gallons of space, and then it's an additional 10 gallons each per goldfish after (at minimum), so two goldfish need 30 gallons of space.

A one gallon tank is not very good for much of anything, but if you would like to keep a Betta fish, you could look at getting a 2.5 or even a 5 gallon tank.