New caretaker.

Hi,
I recently bought 3 goldfish for my daughter. I did some research but
screwed up a bit. According to this calculation
http://www.petlibrary.com/goldfish/examples.htm I have 77 square
inches which means I should be able to support 2 fish with loads to
spare but 3 is pushing it.

What other ways can I help oxygenate the water? Use a bubble blowing
filter?

Also what other kinds of fish can live with goldfish? For example can
guppies? Can axilotls? Thanks in advance.