As Clerk mentioned: They do not have scales. That makes them more susceptible to the environment in the water. That means the water has to be very stable. The only way to truly have a stable water environment is for it to be up and running for several months.
The size of the loach at time of purchase is not important at all. It is a personal preference thing and sometimes a financial thing. Many fish are more expensive as they get larger.
You do need to keep in mind that they get large as adults, which is why you need a large tank to house them. Using a tank smaller than recommended leaves the fish open to problems from stunting.
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