Haven't read all the other posts, but CathyBogart is right!

Goldfish are coldwater fish and bettas are tropical. They should not mix.

Goldfish also need atleast 20 gallons for the first goldfish and 10 gallons for every other one. They also need a heavy duty filter since they produce so much waste, and that could cause problems for a betta. Bettas are also must faster, and although they have different diets, they would most likely eat the food before the goldfish would.