First off, get through the academy and get your self hired. Get through the FTO (field training) program with a good record. Work on the road and make probation, (generally 1 year)
Apply for the team. Our team has tryouts. They do the Cooper PFT. Running, pullups, pushups, vertal jump, and bench pressing 1/2 your body weight. Then there is a psychological exam. Finally an oral board. All members of the team have to vote yes, one down vote and you are not there.
Smaller stature can be an advantage. Most entry team members are small, quick guys.
Physical fitness is a major part. But the psychological makeup is extremely, and to me the single most important factor in a well rounded individual. Shooting, physical fitness, psychological fitness for the stress/quick decision making of the task.
And no, other than the largest metropolitan areas SWAT is part time, on call. Some get incentive pay, others don't. The pay is minimal, if the pay is an issue, you don't need to be there.
Rocky 911's are the issue boot of our team. (last time I checked, they were the boots issued to the 1st SWAT, LAPD's as well)
HiTech do not hold up well. Bates new style boots are not comfortable. (exception the 924 SEAL model, I wear them at work, great boots)
Don't worry about boots just yet. Get there first.