I dont know about your twenty.
The thing about a 5 gallon tank, as I understand it, is that in that amount of water, changes can happen rather rapidly. Certainly temperature fluctuations can be an issue.
In My oppinion, if you 'cycle' the tank, are mindfull of keeping it at a reasonably stable cool temperature, and do your the water changes very regularly. You'll be fine.
As far as temp goes, I mean keeping it in a cool but not drafty location, and Out of any sun. I don't knowwhat chillers are or if they are even a possibility on a tiny tank.
Some people might think a 5 gallon doesnt allow enough swimming room for white clouds. My oppinion is that it is big enough. Unless....
they start making baby white cloud minnows,
which they are prone to do.
As long as you are prepared to deal with the fry, either using them as food, or otherwise , I'd say you'll be fine.
White clouds are Great fish to have IMO. Possibly my all time fav.
They are hardy, as far as I know, and altogether a great time. Java moss will up the odds of having fry