I have a guppy, and he used to reside with two other guppies, before they got sick and died. As far as I know, he has never nipped my female bettas fins. So I guess it might depend on the fish.
Valentine ∴ Male ∴ Light Pink Butterfly Cambodian ∴ VT ∴ 2.5g
Twilight ∴ Male ∴ Cream Butterfly Cambodian ∴ VT ∴ 10g w/ Celestial Fleet
The Celestial Fleet ∴ Neon Tetras ∴ 10g w/ Twilight