german shepherd is the most popular, belgian malinois is gaining schutzhund popularity fast, due to answer to your second question. yes in a way they do. they have to have high amounts of prey and/or play drive (some say prey and food drive, others call it play and food drive, probably other terms too), but basically the higher drive and energy they have the better, which is why the malinois is gaining popularity among schutzhund trainers/competitors. now no one get offended, I love the gsd, but the average malinois has much higher drive and energy than the average gsd, doberman or rottie. just about any breed can do it, but of course the breeds with tendencies to protect are better. however just being protective is not near enough, they also have to be very willing to please their owners or in other words they need to be obedient. especially due to the fact that most corrections lose points, almost any hesitation to obey loses points and so on. and most dogs who train for schutzhund have a hard time even getting a schutzhund I. for the most part those handlers/owners dont mind and just do it for the obedience training, fun and bonding that results. Im sure they mind to an extent, everyone wants their dog to win when they compete but most of them have a lot of fun at it.
but technically aggression is not a requirement. several labs and chesapeake bay retrievers have recieved sch III titles. and every titled schutzhund dog I have ever met I would consider safer than most any other dog I've met. Just consider this, its not about training attack dogs, not in the least. Yes protection training is deffinetly one of the phases, but the dog HAS to be under control at all times. Any sch trained dog should be able to attack the bite sleeve for all they are worth, and 10 seconds later be hugged and wrestled with by the agitator/bad guy. If a dog has a schutzhund title and the agitator cannot play rough with the dog in complete safety (other than drowning in kisses and maybe claw scratched) the dog was titled by one of the irreputable judges and should never have recieved that title.