If you do get a 20 gallon, try and get a 20 long. Cory do better in a tank with a bigger foot print then one with a lot of water because they are bottom dwellers. I currently keep a 20 long that is heavily planted with 9 corys, 8 ember tetra, a betta and an infestation of bladder snails. It's on the higher side of stocked but everyone is happy and healthy. I just introduced everyone slowly. Starting with the plants and the snails, then the betta and half the cories, then the tetras then the rest of the corys. Stock slow and you shouldn't have problems. Also aqadvisor.com/ is super helpful.
There is no such thing as ‘Rescuing’ a betta or any fish for that matter from any LFS. Be it petsmart, Petco, or exotic and aquatic’s down the street. You are not ‘rescuing’ anything, you are buying a fish. It doesn’t matter if that fish has fin rot and you are saving it from dying a slow death, you are still supporting an industry that thinks it’s okay for these fish to live in small pint size bowls.