Have you tried feeding the betta then throwing in some pellets for the corys so he is eating and distracted when the corys eat?
If the corys stay away from the betta most of the time I would keep an eye on it, but if the betta constantly bothers the corys or the corys just keep getting in the way of the betta you might want to seperate them cus its no fun if both cory and betta are getting stressed a lot.
Do you think it could be due to lack of space in the tank? (not sure the size of your tank)
But neways, let us know how you get on. :)
1 Doberman Mix: Emmy, 2 DSH Cats: Rowan & Jacob, 1 Bearded Dragon: Rainier (aka Mista Lizad Man), 1 Eastern Box Turtle: Greta, Two 5g tanks: Alchemy (Halfmoon) and Custard (Yellow VT)