really the best way to have bettas and goldfish "together" would be with a large tank for the golds, and betta tanks on either side of their stand, that way they're not in the same tank, but in the same display.
i personally don't think it'd work aesthetically either, the color and shape just kinda clash (to me at least) so i'd use separate display setups, rather than display their tanks together.
So remember, when you're feeling very small and insecure,
How amazingly unlikely is your birth,
And pray that there's intelligent life somewhere up in space,
'Cause there's bugger-all down here on Earth.
Loki/Grendel/Gail/Beowulf/Camilla/River/Bathory/Arwen/Lafayette/Deb/Jack Skellington (soon)