I wouldn't put a dwarf puffer in a 5 gallon, the water quality is too hard to maintain and they need extremely good water quality.
You're pretty much stuck with either a betta or some kind of killifish. Not much to work with in a 5. Killifish come in hundreds of colors and some are extremely beautiful. You could put up to two in a 5, maybe 3 if they are very small. They also do fine alone unlike smaller schooling fish such as danios, clouds, guppies, etc.
As far as your 20 gallon, is it a long, standard, tall? What are the dimensions? If you want to go with a centerpiece fish, and it's either a standard or a long you can go with some kind of gourami. The pearls are beautiful and reach about 5 inches. Here's a pic:
http://www.aquahobby.com/gallery/e_leeri.php They look better in person. There are also other species that look nice and get about the same size.
If you want to go with a community tank, and it's either a standard or long, you could go with a dwarf gourami, a few corycats, and some small tetras.
If it's a tall you're limited because you have to go with small schooling fish such as neon tetras, danios or white clouds. Or one fish that doesn't move much and doesn't utilize the surface of the water much.
