Let me get this straight...FEMALE
betta with FEMALE Guppies right? If any of them are males then it is a definate no-no. I probably would not normally recommend this, but if all are females you MIGHT get away with it. If you try it though you need to supervise very closely, and don't count on it working. If you notice any nipping or aggression above normal behavior then you need to have another tank you can move the Betta to.
I think that, since all are females, there is a possibility of it working, but female Bettas (and Guppies for that matter) can be nippy and even aggressive so it would be a risk. I won't discourage you from trying it though if you believe all are the fish are peaceful and you have an extra tank just in case.