I'll second the dwarf puffer idea but make sure to fishless cycle the tank before adding it and don't try keeping any shrimp or anything with it as the puffer'll kill them. Also, the 5 gallons minnimum is for when you are keeping several dwarf puffers together. If you don't provide them with this amount of space and you have 2 or more, they are liable to kill each other.
There are some killifish you could also keep in there (but you'll have to research them yourself), as well as some wild betta species or a couple of (female) mosquito fish (heterandria formosa which doesn't even grow to 1") though it's less than ideal for the livebearers as they'd preffer more swimming space. Obviously, not all at once and you should plant the tank heavily for any of them.
That's another point - a heavily planted tank with various bright-colored shrimp can look stunning. Do consider going this route.