The betta will turn the guppies into shredded meat in a matter of minutes. The only way you could keep guppies with a betta is if they are a wild strain, and that's a maybe.
I keep some otto cats with my betta and he still harasses them. But they have a chunk of driftwood to dive into and hide if he's in a particularly territorial mood some days. cory cats and some Kulie loaches i've also never had a problem keeping with my betta as long as they have places to hide. which they should have anyway. and these fish also need filters. any mid-top water fish is going to get roughed up/killed in my experaince.
well i ended up putting the beta in a fish bowl for now instead of his 10g aquarium cause i brought home some feeders that i wanted to try and make sure they werent deseased of some sort and i cant get these little guys to make babies!!!
I kept a betta with many fish (red tail black shark, numerous guppies, etc) and no problem whatsoever. Might depend on tank size and hiding places (as already mentioned).