Most danios, barbs, and tetras would work out fine. I would try to keep twice as many of them as female bettas, though. With the girls themselves, the more you have the better. You need at least 4, and each additional one means that any aggression is that much more diluted.
Female bettas are must faster than males, so I might think that neons could get picked on more than a slightly larger tetra would. Cardinals have the same coloration...you might want to consider them.
Also, here's a shameless plug for corys!
Make sure you have enough filtration!
Good luck, and have fun!