i used to have bettas (only one at a time) in my 10 gallon molly/platy tank but they kept dying, i think it was the stress of so many fish making them susceptible to infections. you might wanna steer clear of fast-moving fish who could over-stimulate your betta.
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)