The further ahead you plan, the better!
How many cories to add at a time depends on the number you plan on having in total. If you were to have 12, you should probably do three batches of four. If you were to have 10, two batches of five would probably be ok. I would add the betta last. They can be aggressive fish, and usually it's a good idea to add the most aggressive fish to the tank last in order to reduce the chances that he'll establish a territory and disapprove of other fish intruding upon it.