The way I look at it , if the betta in a store is being kept in with other small fish and has intact fins, is generally not hassling it's in store tankmates, and looks healthy and alert ect, then it should be fine being kept with small safe fish like Endlers or Harlequins. It will be used to company in the mid to top swimming levels .
If the betta has been kept in a cup or tank alone or you know it has been raised alone for all or most of it's life, then either stick to cories or inverts only for the bottom and no other tankmates ( added first of course ) or keep the betta alone.
Hi
Thats not always an indication though. It depends on how long those fish had been in tank together? We all know the in general new bettas tend to take a few days to settle into an environment. Problems may arise once the male betta has settled and decided to call the new place home. Also, the size and layout of a tank can play an important role. A large tank, heavily planted may provide more cover for other fish, as well as providing the betta with its own territory.
I would hazard a guess that the majority of male bettas have been raised singularly upto the point of going to LFS that then put them with other fish.
There is an lfs near us that, on the few occasions we have been there, has had bettas ripped to shreds and even dead. In two visits to the lfs, we have seen 6 male bettas. Only two of them were untouched by tankmates, the other four were either shredded, being shredded or dead!
Its a lottery really? Some will be able to co-exists others wont. It really depends on whether you want to take the risk of having damaged fish. If it does work out great, you have a nice centre piece in your tank, but if it doesn't then you have a trashed fish or fishes and nowhere to put them?
It comes down to the individual fish keeper, personally i wouldn't keep a male with other fish even though i know people who do.
The only issue i have with keeping a male betta with Cories, is the individual species requirement. Male bettas prefer a much higher temperature than cories, and cories like a fair amount of movement in the tank. A lot of male bettas do not like this movement due to their larger fins. We found this out when we attempted it. We managed to create a nice flow for the cories, whilst having a quieter area for the betta, but having the temperature low enough for the cories played havoc with the bettas fins??
Its through these types of experience that we now house them individually and if we want something else we set up a new tank, like the Dwarf puffer species only tank Netty has just set up.
Hope this helps