agreed with above. however ive never ha a problem with bettas and guppies. maybe ive been lucky with my bettas and their temperaments over tiem, but the above scenario would be ideal if youd like to try this approach.As a general rule male bettas should not be kept with guppies as the long bright tails do tend to antagonise the betta and goad them into nipping and attacking.
On the other hand if you get your betta from a store who keep their males in with guppies , and all the fish are in good health with no ripped fins, then a betta from such a store is more likely to be ok with guppies as he will be used to them.