If your desk can take the weight of a ten gallon then get a ten gallon. The bigger the tank the more fish you can have and the easier it is to get filters and heater for them. More water means the parameters are easier to keep stable ( this doesn't mean you should do less water changes though ) and thus healthier fish are more likely.
In a ten gallon you could have a nice little community. But I will warn you most of the usual tetras like neons, cardinals black widows ect, and most of the commonly found Danios, aren't really suited to that size tank as they are schooling active fish, and like a good bit of length in a tank to swim up and down and display to eachother ect.
Endlers are beautiful little fish. A group of all males would work nicely in a ten gallon ( I say all males so you wouldn't get a population explosion ) Same as a group of all male guppies. Both of these species are bright and colourful, active enough to be entertaining and so on.
A group of female bettas would work in there, 5 or so is often reccomended for them, too few and they tend to squabble amongst themselves a lot more.
A dwarf gourami or pair of sparkling gouramis would also be nice, but you MUST make sure your filter is properly matured and the water very good, as Dwarf gouramis in particular are known to be a bit weak and sensetive.
For bottom feeders, normal cories won't have enough space as they are also social fish and prefer bigger groups and need more space than ten gals. If you have a decent filter then a gaggle of 8 or so Pygmy cories will work fine with a group of other fish in the upper levels ( although pygmies also spend more time off the bottom than other cories ) .
Shrimp are great, Ghost shrimp are active, come out into the open a little more often than Amanos, and are generally quite entertaining ( mine steals food right out of the fishes mouths and runs off with it !) a group of them would do a good job of substrate cleanup. Cherry shrimp are also nice.
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Yay! Since its 40 lets add in a bala shark! Genius! /endsarcasm. 
