If you look at your three white clouds from above, is one significantly slimmer than the other two? If so, "slim" is the male and you have a possible breeding trio. He may also be a little more colorful. They would fit well in your 60 Liter (sort of 15 gallon) tank.
A reverse trio is ok too. The males are more interested in breeding that egg eating. A tank full of white clouds is beautiful, but save your money. It is sometimes better to spawn fewer breeders.
You could spawn more adults, but would have so many fry you would lose a lot of them. The fry are like little slivers of glass and have very small mouths. Like many fry, they gravitate towards the surface and especially feed there.
If you are raising live foods such as paramecium, vinegar eels and Moina or have a good supply of one of the powdered "rotifer" sized foods, you could probably raise up white cloud mountain minnows. Unless the fry are in very shallow water, micro worms that sit on the bottom are useless as food. There is an outside chance that you could feed them a tiny bit of powdered egg yolk at a time. Try gently squeezing a ripe sponge filter over the fry too - there is a certain number of microscopic creatures that thrive there.
Acrylic spawning mops are used to spawn fish whose eggs have little filaments that catch on plants or mops. I do think eggs in some cases would adhere to the mops, but mops offer no significant food source. If you were to fill your tank with the fast growing hornwort, broad-leaved water sprite and/or "Anacharis". Java moss on the bottom would be a treasure.
Tiny fry like them will will fine some food on the plants you don't want the hundreds of fry the 18 breeders would produce, there would be less forage to go around.
Light might bother the developing fry. If you have an aquarium light or shop light hung or draped over the tank, turn it off for the 1 1/2-3 days that it takes for the eggs to develop. The newly hatched fry will just hang there on plants or the water's surface. When they are moving around first feed them. (That is why plants are useful too in that early hatchlings can graze the plants.)
I have bred them outside in container gardens (tubs of 15-40 gallons / 80 to 160 L. in the summer. Although white clouds come from a sort of temperate climate, like many North American and UK fishes they seem to spawn as the water gets warmer. Indoors that could be as low as 21 C/70 F. Certainly they were spawning in warmer water outside. Water temperature accounts for the variable lengths of time the eggs take in hatching.
So long as the parents were in the tub, no fry survived. (I now know one should feed the breeders very heavily when in the tubs.) When the parents were pulled, a dozen fry grew up and were brought in when fall rolled around. One wonders what would have grow up if lots of appropriate live and dusty foods were fed.
Perhaps you could condition your male(s?) in the 40 L container. Also run a sponge filter in there. In the 60 L container condition the female(s?) and also run a sponge filter. In addition to the usual foods you feed them include a feeding of some frozen food after you have defrosted a little bit in Luke-warm water and rinsed it through a fine-meshed net or sieve to remove the organic soup in the frozen stuff that we really don't need in the tank.
If you have access to bite sized live foods (small worms, Daphnia, insect larvae) so much the better. Try to give both containers weekly 50 % partial water changes with previously treated water that has sat open a day or more. Place the male in with the female after such a water changes. Feed them live food if it can not hide someplace in the container.
I don't know if fish shops sell small worms where you live. Here it is useful to put a soap-less jar 7-10 cm wide or an inexpensive glass bowl on the tank bottom. Using a pipette or turkey baster (a large kitchen pippete) 5-15 tubifex, white worms, grindle worms or black worms are placed in that container. Many fishes will prefer the worms to any eggs or fry available. And worms (along with Daphnia) are rich in lipids, which are used in forming eggs.
After maybe three days of that remove the adults and begin your wait for the fry.
This time of the year in the US we get storm fronts (rain, snow) which bounce around the barometer. It is truer of Corydoras, cichlids and Bettas, but those storms often stimulate the fish into spawning. Likewise I could depend upon summer "thunder boomers" to provide spawning fish we could show to our children. If a storm or at least significant precipitation is predicted for your area maybe it would be wise to unite the pair the day before the storm.
The more plants, the more space, the more food organisms, the more frequent (modest) feedings you offer, the better the water quality, the more white cloud fry will have grow up.
Good luck & all the best!