One nice, colorful male molly (perhaps a sailfin molly) or swordtail might be a nice alternative. Peaceful enough to be fine with your other fish but not too large to dominate the tank. You could go for a M/F pair or M/2F trio but then you'd have to make plans for the babies that would inevitably result.
Many websites suggest small amounts of salt in the water if you are keeping livebearers. While generally seen as helpful, it is less necessary if you have hard water than if you have soft water. If you do decide to use salt, use sea salt (the same stuff saltwater aquarium owners use), not the generic pet store "aquarium salt" which is nothing more than glorified table salt.
EDIT: another option would be something like a Golden Wonder killifish. Though be warned, at adult size (about 4") they will be able to eat anything in the tank the size of adult neon tetras or smaller. (I learned that the hard way, when a pair of Golden Wonders over the course of several weeks ate my entire school of neons.) But they are beautiful fish and tend to stick to the top part of the tank which is often the hardest part of the tank to see fish in. Most descriptions of these fish say they prefer soft and slightly acidic water, but I kept them very successfully in pH 7.2-7.4, medium hardness water for years without any trouble.
