The reason why we recommend 20 gallons for the first goldfish, is for measures only. A 10 gallon is very thin and very short, and small allover. If you had a 20 gallon to begin with, you could even keep two goldfish in that

Or a goldfish and a pleco, or some small coldwater fish. But adding guppies with goldfish is not a good idea and won't qualify as "company" to the goldfish. He'll think of them as food, and treat them as such

I had my two last guppies eaten by our fat goldfish when I was little, for the company reason... xD
I think you should aim for the balloon mollies or some other small fish

Platies will also enjoy the same water quality as balloon mollies and other types of them

Swordtails are also nice, but they shouldn't be mixed with platies since they will breed among eachother creating strange hybrids. If you'd choose guppies, you could keep many more of those in the 10 gallon than of the other fish mentioned. Note that they'll breed often and a lot, so give/sell the offspring to a pet shop
