everything i've read says that guppies are just too much of a risk in a tank with bettas. the small ones can become food, and bigger fancies have bright fins that look enough like another betta to prompt an attack. even if you don't have those problems, they're sometimes known to be nippers, which can be bad for a much more slow-swimming fish like a 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)