Your betta should be fine with female guppies. In my experience, you don't want to keep male bettas with male guppies too much, depending on their disposition because male guppy finnage is more showy and could make the betta want to compete with him. Also, even though the OP didn't mention female bettas, never keep a male betta with a female betta. He will kill her.
The male guppy finnage encouraging competition is wrong. The Betta will keep tasting the males tails, because that's about all they can catch. Housing a male Betta with female guppies isn't an issue because the females are too fast for the Betta to catch them.
Also, no, a male Betta will not always kill a female Betta. Both male and female Betta personalities and dispositions vary greatly. A couple times while breeding Betta I had it turn into a best buddies situation. They didn't spawn, but instead hung out together. They didn't even compete for food. They actually begged together. I was curious how long they would co-exist, so both times I left them together. If I remember correctly I needed the tank and separated them after about a month. Granted, this is not typical, but you can't make blanketing statements. The males are not always the the aggressive ones. I've had some nasty females that didn't get along with any other fish. I've had several expensive males killed by females. During spawning, if a male gets too a aggressive the female can jump out and seek protection on top of a floating leaf (and jump back in when she's ready - remember they can breath atmospheric air). Males are not capable of doing that if they are a long finned variety of Betta. That, along with the fact that females are faster, sometimes males are unable to escape.
