I'm sorry but you have been misinformed. Clown loaches DO NOT only grow to the size of their environment, they grow to their normal size. They are unsuitable for long term housing in small tanks, and need re-homing as they get larger and outgrow their tanks.
For small tanks, a far better predator (assuming copper has not been used, and no loaches are present) is the assassin snail. These do not reproduce as quickly as pest snails, and eat snails.
However if snails are reproducing at a high rate this is indicative of another problem, it means that you are overfeeding. If you don't overfeed snails grow and reproduce much more slowly as there is less food for them.
IMO Animals of any sort should only ever be purchased on their own merits, and not for a task they may be seen to perform. They should also be well researched and you should be sure that you will be able to take care of their long term needs including been able to rehouse species that may one day grow too large.
Ade