I think it depends on tank size. At first I was shocked to hear that my lfs was saying it is alright to keep guppies and bettas together, but the tank my friend has with a male betta and three male guppies all get along great. It could just be the betta is nice. He has a cave to hide in when he wants to get away from the other fish and I think the little group of guppies makes the guppies more even with the bettas size. Again, could just be the tank I saw was an exception.
just because youve seen it happen once, doesnt mean that you should tell oithers that its okay to do that, as they are not compatible and tank size is not a big factor in why they arent or are compatible. honestly it all comes down to personality. some bettas will go homocidle and some are very accepting of other fish. the only way that tank size will have an effect is if the tank is 40-50 gallons, as this is enough space for a male betta to establish a proper territory. in which he stays in one area, and doesnt come over into the other area as often, meaning that the guppies know where they should stay in the tank, but when he does find them he will attack as he would when another betta is in his territory