You could try it, in that size of tank I would suggest getting atleast 5 more (to bring the total up to 6). The more female bettas you have, the more the aggression is spread out, and in that size of tank you could have a total of maybe 10 or so (if you didn't add any other kind of fish). You might run into problems with the guppy, the females might pick on her. But you could always try it, and if it doesn't work then move your guppy somewhere else.
You could add some bottom dwelling fish, such as a group of 6 cories, or a group of 3 or so kuhli loaches, or a group of ottos (of atleast 3). But that will mean that you can't add as many females, but you might want something different in your tank besides just the bettas.
. But I will be much more careful this time
