They should get along fine. Remember that swordtails (males in particular) are really good jumpers so make sure you've got a good hood on the tank. Also, you'd want a ratio of 1 male to 2-3 females, so that the male doesn't constantly pester one female... it spreads his affection out so the females can get a break. The crayfish should take care of any fry, so you shouldn't have an overpopulation problem from the females.