Your probably right FM12, but here is my two cents anyways:
Like stated before: goldfish cannot live with a betta, because of temp and filtration needs. One fantail goldfish needs 20g+10 for each additional fish. Comets/feeders need 80g each. 12 would need a 1600g POND. For tank mates for a betta, a ten gallon is needed. Neon tetras are known to nip fins, and Cory catfish need a sand/small gravel substrate AND need to be in groups of 4+.
If you ask me, I would get a 5+ gallon and divide it. And get another betta. 0.5g is WAY to small for a betta. 2.5 is the min. They need heat as well.