I think you could get away with 4 or possibly 5 of the pygmy version if you can find them, my local fish suppliers don't sell pygmy's. You really are limited by your 6 gal tank though for regular cories (People reckon they need to be in schools of 5 or more to be happy) Have you considered maybe getting a larger tank? Then you can add a mix of fish.
If not then just keep him on his own, he has his own little kingdom. Having more fish in there can cause more stress for your betta anyway. If you want some more action then try get some Pygmies. I have only seen them online here in the UK, so you'd get charged for the delivery. I don't know where your located, but it seems they are more common in the states in shops.
It's very tempting to get too excited when you think about adding more fish to a tank, but the best thing if you want lots of fish is larger tanks. My other one is 55 gals and I have lots of different fish in there. I have not had luck with bettas in it though. Larger tanks open up all sorts of more considerations, like avoiding fish that could harrass the betta and nip him or chase him, or him chase them. I kidna wish I had got a 20 gal instead of my 10 gal, it usually only costs a little more. Oh well
I am sure he could live a full and happy life on his own though, king of his own little kingdom where it's peaceful, but having the pygmy's would add some more activity to the tank and a nice peaceful companion, providing he doesn't get grumpy with them, but I doubt he would.