This can be a bit of a sensitive subject when it comes to stocking aquariums.
While its true the hoem aquarium will never compete the natural wonders of natures, the hoem aquarium can be set up very nicely and usually stocked with captive bred fish which has never been to their natural environments be it rivers, lakes or oceans.
Though it has to be said, this is why forums like this are ongoing, to help those in need of advice and experience of those who are experienced in this fishkeeping hobby and we all try to help whether its just a little bit of advice or a lengthy post with lots of information. Ultimately its dfown to each individual to research and ask advice on what may be the best course of action to take.
In this case, a 20 litre / 5 gal tank is a small tank and is in fact too small for most specie, even neon tetras despite they being pretty small. It is not generally advised to stock with shooling or shoaling fish in a tank with such a small footprint.
So therefore those of us who made the mistake of stocking the wrong species will say best not to as the fish are likely to just survive but not thrive and at worst die due to stress or illnesses that occured because of the stress or poor water quaility as its mroe difficult to maintain good water parameters in small volume of water for those inexpereinced in fish keeping.
I myself have had endlers with shrimps ands snails in a 5 gallon tank and thought they were fine. That was until I moved them into a 10 gallon tank, wow, talk about the differences in behaviour in the endlers, they were MUCH more happier as their colours soon became much more vibrant and they swam around much more rather than swiming in more or less the same areas all the time. These endlers are now in a 25 gallon / 95 litre tank with danios and rasboras.
So it takes time to realise what others mean by their recommendations in why samll tanks are not so suitable for most species despite their small sizes. This is why advice fromt he more experienced keeper is worth considering and researching upon in most cases. Though of course there are folks who will recommend the wrong type of advice based on their experiences but not always the right ay to do things. We all learn by different methods and practices.
In the case of neon tetras for example, they are soft water fish and are not particularly 'hardy' menaing they are suscepticle to water parameters and easily prone to diseases. Not only that, the very minium tank size, according to Seriosuly Fish is a 15 Gallon tank, though I would actually lean twoards more a 20 gal or at least a 2.5 feet long tank MINIMUM as they are skittish and can suddenly dart into the tank glass when spooked resulting in injury or even fatal.
Neon Tetra - Paracheirodon innessi
And cories, being a shoaling specie, a VERY sociable fish who requires groups of at very least 6+ though imho preferably at least 8+ (they shoal in their hundreds if not thousands in their natural environments). Again, a soft water specie that require a miminum tank size of 15 gallons for most secies, even for one of the smallest cory specie, Habrosus Cory aka Salt & Pepper cory, one of my favouraite species of all time requires at least a 2 feet aquarium.
Habrosus Corydora - Salt & Pepper Cory
So this basically leaves us with Betta Splendens which is perfect for a 5 to 10 gallon tank. They are lone species that are kept singly mainly due to the simple fact they will fight with other Bettas.
This link explains far better than I can about Betta Splendens -
Betta Splendens Caresheet
And lastly, shrimps and snails are ideal for a 5 gallon tank as well. Though they tend to be more sensitive to water parameters, so care has to be taken to ensure water parameters stays stable throughout. I would recommend Red Cherry Shrimps for the beginner to shrimp keeping.
Red Cherry Shrimps
BTW I usually do not recommend keeping bettas with shrimp simply because more often than not, bettas will make snacks out of shrimps. Exceptions do occur but not a gamble I'd take for the new keeper.
http://www.planetinverts.com/Red Cherry Shrimp.html
I hope that explains things a little more