Well, a basic community will fill your tank up nicely and make it look bigger than it is. Another benefit to a community (when it is put together right) is that the animals do not get in each other's way (ie swim on different levels) and create a nice 'tank ecology'. The typical community consists of a few 'centrepiece' fish, a handful of schooling fish and some scavengers, who eat detritus on the ground (eg uneaten food, algae and dead plants) and turn them into fertiliser for your plants.
I'll use my setup as an example. It's a 20g of similar proportions:
3 Dwarf Gouramis (Centrepiece)
6 Zebra Danios (Schoolers)
2 Otos (Algae eaters)
1 Batik Loach (Scavenger)
1 Short-finned/Plakat Betta (Just an uncategorized resident)
You will find that certain groups of fish fit nicely together in 20g (give or take a few g)
Centrepieces
-Dwarf Cichlids (i.e. apisto agassizi, kribensis cichlids, rams)
-Gouramis (I recommend dwarf gouramis or something of approximate size)
Schoolers
-Tetras (small ones eg neons, cardinals, penguins. No congos, etc!)
-Danios (again, stay away form anything over 2 in. adult length)
-Rasboras
*** I am not sure about rasboras, but tetras and danios should be kept in schools of at least 6 to prevent them from bullying other fish!)
Scavengers
-Loaches
-Cories
-Otos
-Snails
***Just make sure that the secies you pick will fit in your aquarium at adult size!
A far as amounts go, the inch-per-gallon rule should serve as a basic guideline, but note: for a 20 gallon, don't add any fish who grow past 3 inches in length!!! The inch per gallon rule varies a little bit according to how well your tank is planted and how much your fishies poop.
If you are really ambitious you can go for a 'biotope' tank, where all the plants and animals are from the same region (almost like a zoo exhibit
). When I didn't know what to put in my tank, I googled 'stocking a 20 gallon aquarium'. Looking up journals of tanks similar to your size on forums is also a great source of inspiration.
And last but not least, don't be afraid to post your stock-list draft on a forum or two and ask for input!
Hope that helped and feel free to ask questions; stocking a tank is a deep topic!
-Kelly