In my 60 L tank, i've had guppies, platy's, dwarf gourami's, apistogrammas, neon tetras, bronze cory's, Cherry Barbs (although not necessarily at the same time). Basically i'd approximate it to around 16Gal initially which means you can keep 16 inches worth of fish, so perhaps 6 average tetras, a couple of larger fish, the apisto for example or platy's and a few bottom dwellers. With proper maintenance you can keep more but you have to keep an eye on the tank that you don't overstock, and be aware that with smaller tanks, if something goes wrong it goes wrong quicker than in a larger tank.
Hope that helps.