i will never understand why people ask, but then again everyone always does!
personally, i'd head out and pick up the Baesch Aquarium Atlas; flip around, get a feel for some fish. i've fallen in love with Australian Blue Eyes (a fish i never knew existed!) and i'm dead-set on getting at least 6 for the 50g me and my dad are setting up
for a 55g, hmm... let's see...
Spotlighters
2 Dwarf Gouramis - 2"
Schoolers
6 Blind Cavefish - 3.5"
6
Boehlkea Frechochui - 2"
6 Emporer Tetras - 2"
Scavengers
6 Leopard Cories - 2"
4 Peppered Cories - 2 3/4"
2 Albino Cories - 2 1/3"
that's simple Gouramis, Characins and Cories. something a bit more difficult, but prettier, to achieve would probably be along the lines of.
Spotlighter
1 Butterflyfish - 4"
Schoolers
8 Celebes Rainbowfish
4
Betta Imbellis - 2"
Scavengers
2 Talking Catfish - 6"
and lastly, simple, easy-to-come buy, but pretty nonetheless
Spotlighter
1 Male Betta
Schoolers
8 Zebra Danios
8 Leopard Danios
Scavengers
8 Albino Cories
=) really, it's all up to you. go with what you want!
(excuse my babble, i got carried away with my atlas -snugs atlas-)