If your filter is adjustable, I would up it. A 10 gallon gives your Betta enough room to get away from the current and as others have noted, current gives Bettas exercise and makes them stronger. An alternative filter would be an internal with a spray bar you can point to the tank wall which baffles but not too much. This one (or the IF-201) is small and effective. Unfortunately, both are out of stock but expected this month.
I've had Cories in with 6+ different Bettas. I don't recall the Bettas even noticing them. You could easily do seven in a 10 gallon which is what I'd suggest. If you add too many fish at one time it messes with the pH.
Adding too many fish at the same time, even Nano/Micro fish, can change the bioload enough to overwhelm the beneficial bacteria and start a mini-cycle. Keep an eye on your parameters if you add more than 5-7 (the number most suggested by long-time aquarists). And as I don't always follow my own advice, I can attest that water changes and Prime will get you through the mini-cycle.
You could get some of each and they'd do fine. Mine don't really shoal that much as they are too busy picking at the plants and substrate for goodies. I have 12 Pygmies and 7 Hasbrosus; Hastatus are smaller than the Hasbrosus and more the size of the Pygmy.
Sorry two of them died; they are a bit sensitive ~ more so than the Hasbrosus or Hastus.
Wait until you get your 20 long and have 15+ swimming around as I do. ;-)
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