The betta would be a big NO NO!

bettas,shouldn't be place in a tank with guppies,or mollies ect ect
The colors of the male guppies are very colorful and they
have long floating fins and tails usally and of course the
betta would have a go at the fins/tails because of the colors.
I would give every guppy 2 gallons each,and maby 3 gallons
each for the mollies i think black mollies is the same aswell.

good luck.
-Black oranda.