It depends. My bias would be towards getting more of the same type of fish, as fish tend to feel more comfortable in larger schools. Also, I think it makes for a nicer display.
Before you add more fish, I'd make sure that your ammonia and nitrites are reading zero across the board, and that you've properly cycled your tank from the beginning.
After that, you could certainly add more guppies, mollies, or more of whatever tetras you have. If you would like to add a different kind of fish, you might consider adding something that inhabits a different strata of the tank-- the fish you have already generally spend most of their time in the middle of the tank, though livebearers I've noticed will look everywhere inquisitively. You might think of adding a few corydoras catfish. These are active and very entertaining.
Whatever you do add, I'd add fish gradually, only a couple at a time. Hope that helps.
