Well, in my opinion, breeding bettas (especially veiltails) just to see what happens is irresponsible, for the exact reason you're stating. There are already too many being used, abused, and dying. What would you do with potentially hundreds of veil tail bettas? VT is a very dominant trait and would almost certainly wipe out the option of HM offspring. Your lfs might not be able to or interested in housing them, and the chains won't take them when they already have cups of dead bettas, you know what I mean? At the end of the day you just have to decide if bringing new life into the world that wouldn't exist without your interference is going to be a positive thing for those fish.
If you are still interested I'd just say do tons of research and save up for a quality pair of fish, and have a confirmed plan of where the fish will go beforehand. Pet store bettas also have unknown backgrounds and oftentimes hidden problems.
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