There are different types of cories. You can get pygmy cories, which are about 0.75 inches long so you wouldn't be overstocked with six of those, but they might be hard to find unless you buy them online.
Julii cories are a bit bigger at about 1.5 to 2 inches. Albino cories are about 2 to 2.5 inches.
Ideally, you should have six or more in a tank, but they can do well in a group of four. I have three in my tank (had four, but one mysteriously disappeared), and they're doing great.
You can get a small number of shrimp, but some bettas are very aggressive, and will attack them immediately. Other bettas might not attack them right away, but once they taste one shrimp, they'll learn that it's a delicious snack. In that case you'll have to have plenty of hiding places, and be prepared for dead shrimp as well as an overfed betta.
If you're interested in getting more low-maintenance plants you can get amazon swords, anubias, moneyworts, java fern and such. They don't require much light, and little to no fertilization. http://www.bettafish.com/showthread....ht=plant+guide
is a great guide right here in the forums for plants. I suggest you stay away from the hornwort as they shed like crazy, and make a huge mess in the tank.
Hope this helps.