I don't see consider Cotto a threat to Mayweather. Cotto lacks defense to an uppercut and you need to watch Chavez-Mayweather to see why I think that Cotto is no threat. Williams and Mosley and somewhat Clottey (style wise) are bigger threats to mayweather than Cotto.
Protected? Cotto has fought Mosley, Margarito, and Clottey since '07. Three of the best welterweights in the world. Some protection. Floyd wants no part of anybody with a pulse over 140 so don't even bring his name up. I don't see people lining up to face Williams so don't single Cotto out. By the way, I don't view Williams as being so impressive as a fighter as much as he is a specimen who poses a unique set of problems for opponents due to his physical dimensions. I seem to remember Carlos Quintana making him look quite ordinary and unable to make adjustments. Finally, I highly doubt Cotto, ever the professional, would duck Pacman considering that it is the latter that is coming up in weight.