By any calculation, 50 CFM is large enough for any of those rooms. Problem is, the 50 CFM fans tend to be the bottom of the line models, and not very quiet.
Panasonic fans tend to have the best noise specs. I prefer 1.0 sones MAX and 0.8 or 0.6 is great! Also, having more air flow will help insure no moisture problems.
You will normally have to do some ceiling repair to replace a fan, but this is not that hard.