What bedding are you using? That would be my first suspect as far as the sniffles go. All rats carry the respiratory disease known as mycoplasma, and various things cause it to flare up.

However, RI wouldn't cause cannibalism. It's almost always stress that causes cannibalism. If not stress, the second thing I would check would be the health of your stock. Obesity raises the chances of cannibalism.

I breed on a very small scale, so I take pretty good care of my rats. Each pregnant mother gets a private bin with a nest box and is given at least 1 month to recover after weaning. Female rats aren't bred until they are 3 months old, and aren't bred after their health starts declining from age (or for other reasons).