Okay, no offense, but why would you cater to the whims of a 16 yr old? Dumping 20 feeder goldfish into the tank that can't support them just so they "die and cycle the tank" isn't doing very much to teach her responsible pet ownership. Those goldfish are going to ruin the water for any other fish by introducing disease and parasites. Second, when you do a water change after the last of the goldfish have died, you will be, in effect, restarting the whole cycle over again because you will have removed the water in which the bacteria was living. Third, the death of 20 goldfish will have the ammonia level spiked so high that a proper cycle would probably take a touch longer than normal because there is too much ammonia to cope with. If you put fish directly into the goldfish contaminated water, the ammonia left behind will be toxic to the new fish.
One of the first things I would do is before you get any other fish is ask yourself this:
Who's tank is it, yours or hers?
If she is going to be the primary caretaker of this tank, then you ought to get her a few aquarium keeping books before she can even comptemplate fish. Make purchasing the fish =her= responsiblity with her own money, that way she will apprieate their lives a little more when $40 of fish go belly up because she didn't care for them properly. Guide her choices, but don't make them for her, and insist that she can tell you at least three facts about the fish she's picked, and question whether or not she feels the habitat and the fish are matched properly.
If you are going to be the primary caretaker, read a few good aquarium books and pick fish from there that you've concluded with your best knowledgable judgment would be appropriate for a five gallon tank.
Fish deserve the same care and respect you'd give a puppy. And just like you wouldn't give your daughter the gift of a puppy out of the blue, please don't make this tank another sad story of neglect because you don't want to take care of it, and she doesn't have the time or interest in doing so because of other, more teenage, interests.