Please help with upgrading setup

I currently have 1 4" inch comet housed in a 10 Gallon tank. I have a
air driven corner filter. (I change the water every week, water quality
has been excellent) I want to add at least 1 other fish, and get a
larger tank - either a 20 or 29 gallon. I have a few questions. 1) Can
I get a fancy goldfish, or should I stick with another comet? 2) What
size tank would be sufficient to house a total of 2 or 3 goldfish? 3)
What would be a recommended filtration setup for the tank? (brand, flow
rate, etc) 4) What is the process I should follow to move from the 10
to the larger tank?
Thank you for your help