No, GF don't eat other fish.
I used to keep 10 balloon-mollies with my GF.
The thing is that the guppies can't the medication i put in for the GF.
Not sure about betta, but i suggest you separate it during medication of your Gf tank.
Betta can live in a glass provided you do waterchange once a week. Remember to cover the top, betta can jump.
What kind of betta you r keeping? HM?
Any photo of your set up?