With your set-up, a single sex group would be ideal and fairer on the fish. Because they care for their young, they can get stressed when there are things around that want to eat the fry/eggs, which includes tetras larger than neons and corys. And yeah, they might all live in the same river system in the wild, but the Amazon Basin is big. A 29g tank is not.
The jury seemed out on whether or not the dwarf corydoras species would be okay dithers, but I decided to not bother and just shifted the apistos.