I really like the pineapple swordtail, the marigold wag swordtail, and the red wag swordtail. All of those species are fairly easy to find, look amazing, and would fit well in your tank.
You may want to get three females though, or else you will probably end up having many swordtail babies. Otherwise, I would go with a male and two females. Also, be sure to watch for aggression between all three of the species you will have in your tank, as they do not always coexist peacefully.
You should post some pictures of your tank once you get your new additions.