There is no written secondary for Mayo. You'll get an e-mail inviting you for a phone interview after they screen your application (MCAT & GPA are important here, but so are other factors). Check the interview feedback for past questions (the phone interview is standardized).
On-site interviews will begin towards the middle/end of September. The admissions committee then meets once each month to decide who out of the interviewees will be accepted, so yes, rolling admissions. Each interviewee is ranked and you will remain on this ranked list throughought the interview season (until April or so) until you are either accepted (bubbled up to the top of the list), asked to join the alternate/waitlist, or are rejected if you are so low on the rank list that you will likely not be accepted.
Typically they interview between 250 and 300 people on-site and offer 80-120 people spots in the class to fill it.
Definite pros on your application would include service committments, either volunteer or paid, especially international experience (but certainly not a requirement). They favor applicants from Minnesota, Florida and Arizona, as well as under-represented minorities of all types. Interview day it is helpful to show an interest in the Mayo Clinic and Rochester (i.e., don't tell them that you hate the cold or you would rather die than live in a small town).
Good luck, let us know if you have any questions.
