I would post something in the betta section. I think 14 liters is around 3 us gallons which is not IMO large enough for neons. I am thinking maybe one betta would be ok in it and thats about it.
You can't put 3-4 Guppies in a 3 gallon...too cramped. Guppies need at least a 5 gallon, preferably a 10. And then you'd have the breeding problem too.
The only thing I can think of for now, would be a Betta, Male or female. But I'll keep thinking...
Another option could be a variety of shrimp. Freshwater shrimp are plentiful in the hobby if you have a good fish store nearby (not a large chain store). Many stay very small and produce very little waste. Like cherry shrimp, ghost shrimp, zebra shrimp, redfronted or rednosed shrimp, green shrimp, blue midget shrimp, and bumblebee shrimp to name a few. They range from bright red to lime green to powder blue. You'd feed them sinking algae wafers. Half of a wafer every 2 days would be plenty.
But yes you could have a couple of african dwarf frogs. Just make sure that they're not african CLAWED frogs. If you have any questions on ADFs I'd be happy to help.
i would go with a betta and an african dwarf frog.
you cant put a guppy in there.it will be too cramped.Guppies like room to swim about.
imo i wouldnt put platies in their either for the same reason.
and endlers are a no as well.they are just the same as guppies.
They get quite large and if you wanted to keep one with a betta you'd have trouble as the clawed frog is aggressive.
I would probably plant the tank heavily and get some red cherry shrimp. Here's a link with info on shrimp to whet your appetite : http/www.petshrimp.com/index.html I know a lot of people don't realise how many different varieties there are or how rewarding keeping them can be. You could try creating an environment where they can breed and multiply naturaly. Not only would they be entertaining but also quite low-maintainance and pleasant aestheticaly.
As for a non-betta fish option, though I would personaly advise against it, heterandria formosa only gets to about three quarters of an inch. You could keep 3 females in a 3 gallon as they are so very tiny. They are livebearers and very hardy and tolerant of lower temps (in most places they do fine without a heater). However, don't get males or a mixed sex group - both because of possible aggression (though these mosquito fish are peaceful) and because of breeding issues. They can be sexed just like guppies and other common livebearers - males have the modified anal fin (gonopodium) while a female's anal fin is fan-shaped. They aren't a particularly colorful or unusual fish but they are pretty much the smallest common species so you could just about keep some in a 3 gallon with some sparse planting and (if you like) a couple of shrimp. Do make sure you keep up with maintainance and that the tank is well-filtered and cycled beforehand as fluctuations occur easily and can cause problems rapidly in such a small volume.