Remember to add the stock slowly, you don't want to throw in a bunch of fish all at once that the biological filtration cannot keep up with. You might want to remove the betta, rearrange everything in the tank, add the neons, and then add the betta last. This will help reduce his urge to defend "his" territory since his surroundings will be unfamiliar to him.
If you're considering cories, make sure they have decorations to hide under, and that your substrate is soft and smooth. If you have rough gravel, they may injure the delicate barbels on their faces on coarse gravel as they are adapted to fine, sandy substrates. You should choose the species Corydoras pygmaeus--pygmy cories. These will stay smaller than other species, so you can get more individuals. They are a social species, so you should plan to get a group of them if you get any at all.
Last edited by Adastra; 10-14-2010 at 02:47 PM.