Actually you probably don't want to put other fish in unless you have a bigger tank. Perhaps a couple smaller ones would be okay but only if your 10 g are set up and properly cycled already. Otherwise all the fish waste will make the water become toxic. The betta might be able to handle it since they are a labyrinth breather.. or whatever the actual proper term for bettas are, but the other fish would quickly succumb. Goldfish in particular tend to produce a lot of waste. And no matter what fish you add, there's always a chance of even a normally docile species being a fin nipper or vice versa with the betta.

Here is an excellent FAQ about tankmates for bettas from TFF:

http://www.fishforums.net/index.php?showtopic=96682