Betta fish! xD.. well, they aren't all friendly to each other, but a male with females wouldn't have much problems (I think..). I just wouldn't ever consider putting two males together LOL..
One gold fish, or one betta. Sadly, there just isn't much you can really do without a heater. It is probably best to wait until you can afford to go with a bigger tank too (29-55gal). I hate to be "debbie downer" but you might just be better off until then. There are just more options, and there is more margin for error with a bigger tank.
I like to see people start off in this hobby I just don't want to see them get discouraged because they start off with a ten gallon and end up making a few normal mistakes.