You won't find anything more peaceful than cories, they are one of very few fish that are legitimately good tankmates for bettas.
Put him in already! Unless you haven't finished cycling the 10 gallon or something. The great thing about bettas and cories is that they are bottom dwellers and your betta is largely a surface dweller, and they don't care one bit about othe fish in the tank. As long as the betta is being moved into the tank with the cories and not the other way around, it should be fine!
Watch closely and leave his current tank set up in case it doesn't work, but this sounds like a good match-up. Good luck!