A fantail in a 10g is fine! The basic rule for Goldfish is 2 gallons per inch of fish. =0)

I would be very sparse with adding anything. Guppies would not do too well, IMO, because they need warm temps while Goldfish like cooler water. I'm trying to think of a good Goldfish-friend, but I can't off hand. Maybe adding a few little things, like a pair of mystery snails or ghost shrimp?