I don't know how you would mix a betta with a goldfish as one is tropical and the other is a cold water fish...But anyway, there are many fish that can mix well with betta, I myself have Platys and Feather tailed Corys in my tank with my betta and there is no problem. Most Tetra will do good with a betta from what I've been lead to believe, as long as they are not to small they think its food. Avoid aggressive fish, as well as really colorful long tailed fish as they might think that it is another Betta and you should be fine!