For creatine, while it is true that the supplement serves to help you push out an extra rep or two, I've literally never heard of anyone getting "overuse injuries" due to starting creatine. It's just a scare tactic on one of the best-studied supplements out there. Read up on it here: http://examine.com/supplements/Creatine/

For breathing: http://en.wikipedia.org/wiki/Valsalva_maneuver

[If it matters, I'm a not very competitive powerlifter who has been lifting for nearly two years]