Short answer
Yes, plain cornmeal is safe for dogs of any size, including small ones. The main risk is overeating, which could cause a mild stomach upset. Offer just a small amount mixed into their food. Watch for any vomiting or diarrhea, and contact your usual vet if you see those signs.
Cornmeal — Food safety facts
Safe to share · Dogs · full breakdown on the linked food page
Plain cornmeal is safe. Cornbread mix usually contains sugar and sometimes xylitol.
The full verdict, portion guidance and serving notes are collected on the Cornmeal food facts page.