What Iggy says
I have always found my bettas did better on their own. But I understand the need to have more bods in there as you are thinking thats really a lot of water for 1 fish. It isnt. But theres no reason you cant try the corys, but get at least 4, corys do better in company and 4 I have found to be a good number to keep them happy. you need to make provisions for the cory though they need caves, plants (not an issue in a planted tank) and a soft substrate so they dont damage or erode their barbels. I find 2mm pea gravel works well with Cory. Another sugggestion of you fancy something unusal would be amano shrimp, they will help keep yuor plants free from algae and are exceptionally pretty and in planted tanks will thrive. The only proviso for them is to provide them a small cave they can retreat to, as once a month or so they shed their outer skin and then they are vulnerable to a hungry confident betta, but if you provide them places to hide form the betta when the groiw their new shell/skin they will do fine. Always keep the water lower with shrimp they are amazing escape artists, I know, I keep losing them
Oh meant to say, dont add the corys until the tank is at least 2 months old, they are really sensitive to new tanks and they require mega clean tanks and special feeding, contrary to common misconception they arent algae eaters and they cant survive on scraps left over from the betta, so they need their own sinking wafers or cory food. They like the betta love worms, daphnia, shrimp (remember the amano, thats why the need a place to hide when they shed their shell), plankton, krill.
Good luck and Hope this helps.