You could possibly divide that 20g, maybe using a clear divider or the DIY craft mesh and see how the Betta reacts to movement on the other side?
That way you could get the Corydoras and keep the Betta Separate if needed?
Well, the thing is, he doesn't react to them at all unless they just happen to swim right in front of him, then he chases them down to teach them a good lesson I suppose. He doesn't flare, and doesn't really show any signs of aggression, he's well fed too so I don't see it being a "gotta eat that guy" reaction. It's more of a toy-chasing movement. So I don't think a divider would do any good (at least for this guy) but in the future if I get over the trauma of this go-round, that's definitely the route I'll take. I'm going to move the cories out tonight, then once the betta settles down and de-stresses put him in a 5 gallon. I'm honestly probably just going to use that 20 for some Mickey Mouse platy's and call it a day for betta tankmates. Better safe than sorry!
Swim Shady: doubletail male
Ella: veiltail female
Neo: crowntail male
Bruiser: crowntail male
"When you see a fish you don't think of its scales, do you? You think of its speed, its floating, flashing body seen through the water... If I made fins and eyes and scales, I would arrest its movement, give a patter or shape of reality. I want just the flash of its spirit" ~ Constantin Brancusi