With bettas even neons tend to be nippy. Please, don't take things you see being done at LFS's as an example of how to do things at home. There are a number of criteria that are so different between how things are done, its not practical or safe for a home environment in most cases.
To begin with, fish in a LFS are very temporary. This betta that is in the tank with neons may have been there for a week or more, but the neons in the tank with him will likely be different during the course of that same week. Neons stress easy, so their time spent in a LFS tank is not the place where they would typically be "nipping fins" or causing any type of issue for the few days they are there.
Bettas at LFS's also usually change almost weekly, depending on location. What can be achieved for a week or 2 is not always the same thing after 6 months or a year... when fish begin to mature and settle into a permanent environment with established territories.
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