Generally a trainer will choose a jockey based on their familiarity with the jockey and his style, and the jockey's familiarity with the horse. You can add or take away anything from this question but the answer is " By the trainers" Familiarity, the jockey's uniform or what month he or she was born. But the answer is the trainer picks the jockey.
Horses are chosen for the Melbourne Cup by the horse committee and the top 24 who can run the distance get chosen. They also needed to have one a few races they have entered. If they have not ran the distance before and they go through, it may be because the owner and trainer have thought the horse can run the distance.
The first leg of the triple crown the Kentucky Derby requires horses to 'qualify' by having graded stakes earnings. Then each horses winnings are calculated and the top 20 money earners are allowed into the race. The Preakness and Belmont are open races and do not require you to qualify.
Diane Crump rode Fathom in the 1970 Kentucky Derby
Plenty of them
No female jockey has ever won the Kentucky Derby. In fact, only five female jockeys have ever ridden in the Derby. Julie Krone was the first female jockey to win a Triple Crown event, the 1993 Belmont Stakes aboard Colonial Affair.
Yes, The Kentucky derby is open to members of both sexes.
Calvin Borel rode Stay Thirsty in the 2011 Kentucky Derby. Stay Thirsty ended up finishing 12th.
Not on a regular basis, usually, although jockeys who aren't "big names" might. Generally a jockey will try to ride the horse in a workout beforehand, and most people will have a regular jockey they use for each horse.
I believe that the horses owners have to hire them
Oliver Lewis [1856 - 1924] was the first African American jockey to ride in the Kentucky Derby. In fact, he rode the blood-red chestnut Aristides in the very first Derby. On that afternoon of May 17, 1875, horse and jockey rode to victory.
Jockeys
The youngest Kentucky rider was an African American teenager 15 years old by the name of Alonzo Lonnie Clayton. He ran many races between 1875 and 1902. On May 11, 1892 he became the youngest rider to win in the Kentucky Derby.
Street Sense [b. February 23, 2004] finished first in the 133rd running of the Kentucky Derby on May 5, 2007. His jockey was Calvin H. Borel [November 8, 1966]. Borel went on to ride the wnning horses in the 2009 and 2010 Kentucky Derbies.
He rode Super Saver and he placed first with a time of 2 minutes and 4 seconds