Bettas and cories usually go well. I tried it and my betta liked them, the planted tank just didn't work with him. He split his fin, and got fin rot from my plants that weren't doing so well at the time (I do weekly water changes, just some of the leaves were dying and polluting the water -drove me crazy as I am a perfectionist
) Then I got a new filter which cleared up the water, and the plants started growing better, but the current was too much for him. Oh well...some things were just not meant to be.
I think that he is quite happy in his 5.5 gal.
I always make sure to spend time with him, give him a little time with the mirror, and ask my family to talk to him as well. He will soon have a buddy to flare at (only when I remove the cardboard between the tanks) in a tank next to him. I personally won't put a betta in anything under 5 gallons JMO.