Actually, guppies and bettas usually do fine together. There may be the hyper aggressive betta like mine that won't allow anything, but for the most part they are okay tank mates. If you can go with feeder guppies or ones that don't have long vibrant fins then it'd be better...but there is recorded success with fancy guppies and bettas.
Here's just one thread of a woman who had to keep guppies with her betta, http://www.bettafish.com/showthread.php?t=65012
If you just google bettas with guppies you'll get a bunch of results. They're even on a few "recommended tankmates" lists.
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