no no no no no way!

your tank is too small for the goldfish alone, you dont want any more fish in there.

Bettas should be kept alone, or with well researched tank mates that need the same requirements.
Goldfish are coolwater fish, bettas need the warmer end of tropical temps. goldfish need string filtration which will create a lot of current, bettas hate current.
So no, please dont do it.