Of course you can shoulder roll punches coming from a southpaw. Any straight punch can be rolled. His shoulder protects him from both jabs and straights from orthodox punchers, does it not? Not that it matters, because he's not married to the shoulder roll. He can adopt an orthodox guard or a few other looks to find the one he has success with.
It is hard to say because you normally dont do the shoulder roll if you are conventional against a southpaw. HOWEVER, If you look at ALL of floyds fights against southpaws, he never starts off fighting using the philly shell/shoulder roll. HE starts off with two hands up in a basic boxing guard. He only goes to the shoulder roll when he knows that he will be able to get away with it and even though he protects himself great with the shoulder roll he is still great with regular guards.
cotto breaks down his opponents but he is not a ko artist. and clottey is strong but ask yourself, does that result in ko's. out of 35 wins, clottey only has 20 koes. hes a solid puncher but not a big puncher. and again, the point you're not focusing on is mayweather's counter punching ability could very well allow him to catch pacquiao on occasion and hurt him, something cotto and clottey couldnt do.