My betta is a big baby, begs for food and lets the tang push her around. The clown and the betta get along great and are never very far apart. My clown has tanken to a shroom and the betta hovers above it.
Be ready to remove the new fish. One problem with using size to tell gender is that just because the one is smaller doesn't mean its female. Since the fish is wild it could be male and just smaller do to a lack of food. Once they change sex they can't go back so if you get two males they will...