I think saving one from your LFS is very kind. I used to do this. My males never messed with the corys, they corys typically stay near the bottom and your betta will stay near the surface. Make sure you turn your filter settings as low as they will go if you can; bettas have a difficult time moving around in any sort of current because of their long fins. They are acclimated to standing water. You could even use the 5 gallon with no filter and add no other fish. Make regular water changes, and you'll have no issues. The main problem with keeping bettas is the person keeping the betta. You can't become lax with your care.
Also, I don't know if you intended to place a female in the tank, but that is not wise. The male will kill her. A female may only be placed into the tank after the male has built his bubble nest, and the two may NEVER be left unsupervised. As soon as they spawn the female must be removed or she will kill the male.