Please don't dismiss what I am going to write, it is meant to offer useable advice.
First, you do not know that the betta is getting along with the cories; none of us can talk to fish, and that is what we would need to be able to do to know this. The fact that physical aggression is not visible does not mean the fish are not communicating chemically, with allomones. These if they are "aggressive" in nature can stress fish out just as much as physical attacks. Now, having said that, it is less likely a betta would be annoyed with cories, but again this is not an absolute. And upper level fish is a very different ball game, as the betta is going to view the space as "his," and intruders may or may not be met with resistance.
You mention gravel, cories should be over sand so they can function properly.