12 white clouds would be fine in a 20gallon and a snail would be a great addition. Mystery snails do tend to go after live plants though, so keep that in mind. I'm not sure if anything needs to be done about the crack in his shell or what you could do, but keeping the pH at 7 or above would be best for his shell health. One of the easiest ways to raise the hardness of your water is to add some crushed coral. It's quite common in african rift tank set-ups... Just a bit mixed into your regular gravel would raise the hardness enough to ensure the health of your snails shell.

The only thing that concerns me would be your water temp. They do best in water that is 78 to 84 degrees, but could probably do ok in mid to low 70's. You may want to re-think this type of snail if your water will be getting into the 60's. Goodluck!