I have one female Guppy in my original tank along with my 2 female Bettas. My girls are still very small and the guppy is much larger so there have been no problems.
I was expecting the Guppy to have fry which never seemed to arrive! Yesterday when I got home I looked at them all and the guppy looked rather thin and the Bettas were looking rather plump! My heart sank because I had removed the guppy into a special breeding box but released her a few days ago after nothing happened because she was getting very anxious locked up.
I managed to save 9 fry and put them in the breeding tank, the rest became lunch for my greedy Bettas :( These 9 are nice and strong though so I'm hoping most if not all grow to adulthood on the nice diet they are receiving!