Can you think of any, or make a case for any? An old school fighter who has more success against Modern Boxers without changing their style? Modern / current rules of course. The one that comes to mind for me is Canzoneri. His tricks and skills would truly confuse fighters today IMO.
Loads and loads. There are more weight divisions so the weight divisions are weaker. I don't think that boxing has got better, really, the best are just the best. Someone like Charley Burley or Sam Langford would have 15 belts or so to chase across three weight divisions, get one, you're in business.