Honestly in that tank I would if i was prepared to keep on top of the water changes that i know would have to do almost every other day i would keep
The Guppies
or
The Dannios
Or
The Tetras
Or
1
betta
The RAMS would need 20 gals not 20 Ltrs
The Gouramis if they are Honeys and only 2.5" fish I may consider instead of the Guppies in that list above.
One thing though I never believe what the tank manufacturer says the tank will hold becaause they use the OUTSIDE dimensions to calculate the volume which I think is a con, but I digress.
When I list the size of any of my tanks that is the volume of water i actually hand counted into the tank after i have put the substrate and any large decorations into the tank.
If you do the same you will have a better idea of the volume of water you have to play with.
the 'general' rule is 1" of fish per gallon of water now that has to be tempered with also making sure that the fish has space to turn round in the tank and swim.
So a 12" fish in an 18" long tank holding 15Gallons is not going to fit.
Always base your stocking not on how big the immature fish is when you buy it and put in the tank but on how big the fish will be when it reaches maturity.