Does shallow breathing for 48 hrs from cracked ribs greatly increase chances of getting pneumonia, or is that too short a time span?

Less chance: You are at a risk of developing pneumonia. But if you develop any signs of fever, chills, shortness of breath, cough with yellow/greenish phlegm, decreased appetite or confusion, then I would highly advise you to seek immediate medical attention.

There is a tendency,: but there are more variables. Recurrent use of warmed ultra-fine-mist humidification, such as facilitated by the MyPurMist device with distilled water to assist the lung mucociliary escalator, as illustrated here: http://goo.gl/IXDw19, would likely also help avoid bronchitis & possible subsequent pneumonia.