I think the betta and the cories are just fine together, actually. 99% of people I know, who have these species together, have great results. 1% can disagree.
Not all bettas are solitary. They actually are social, believe it or not. Rearranging the decor, after removing the betta, and perhaps giving the cories more to hide under and in would be the best way to go, then reintroduce him. Whenever adding fish to a betta's domain, you need to remove the betta, and rearrange items to ensure it is no longer his territory and he has to accept the other fish, and make a new territory.