Cories are such sweet fish - I have four...2 of them are 5-6 yrs old, one is 4, and the other 3. Just make sure they have a lot of hiding places, and at least 2 of each type. They also love it when you soak their food tabs in garlic, and it will prevent them from getting some diseases.
And I guess it depends on the personality of the betta...my little Wantai loves his cories and never bites them, while other people say their betta fish are more agressive. Feeding the cory cats at night is a good solution because that's when the betta sleep.
Also, you don't necessarily need sand - we use gravel and they do just fine. If you use gravel just make sure it is smooth so it doesn't give them barbel irritation.