I think Corydoras are wonderful and am a total advocate for them towards anyone considering a tankmate for their betta. Honestly, I think they would only do well for your tank should you choose them over the Otos. As Mister Sparkle said, though, they do best in tanks of 10 gallons or more and if remember your tank is 30L, right? That's just shy of 8 gallons so I'm not sure if Corydoras would be a proper addition - maybe you could get away with the bare minimum: three?
There are other options, however, should you need alternatives. Tetra, Danio, Guppies, and other fish I have no experience with; I just know they can be housed in an aquarium of your size. I think...