That depends on the temperament of your Betta. Some Bettas are gentle, but most are not.
Even male and female Bettas fight, so be careful.
Platys, mollies, and African Dwarf Frogs *can* be added to a 10 gallon (minimum) Betta tank, but I prefer to keep my six Bettas in their own bowls and tanks (1 female in 1/2 gal fishbowl, 1male and1 female in a divided 2 gallon bowl, 1 male in a 5 gallon, 1 male in a 3 gallon, and 1 male in a baby biorb with water table down to just above the filter output).