lots of questions here and probably lots of different answers too.
gearing is somewhat dependent on the terrian in your area, and the type of rider you are (your fitness, strength, spin -vs- mash). i have seen ppl run 34:17 and 42:18...its really a matter of preference.
the classes are generally A, B, C, masters, juniors, and sometimes there will be seperate ss class(es). this depends on your area. out in seattle and portland there are alot of ss-ers so i think its more common out there than it is in the northeast. but it doesnt really matter because you can still just jump in the c's or the b's, depending on your abilities.
cx races are timed, so depending on your class it could range from 30 min + 1 lap, to 60 min + 1 lap. timewise, one lap can range from approx 5 to 7 minutes.
for most races you will need a license, but you can get a day license for about $5 if you dont want to buy a yearly license.
best thing to do is just go and start racing. be prepared to get schooled, but dont worry about it. you will have a blast. cx is a great sport, especially as ss!
I'v never seen a SS class but I've seen som SS'ers kick some serious ass. I would not think that fixed is a good thing because of mounting and dismounting as well as obstacles.
Last year I attended the Crank Brothers sponsored UCI cyclocross race at the Alpenrose dairy, in Portland, Oregon. Some lady was racing on a regular track bike with no brakes and narrow, slick tires. The course was a complete mud bog. I don't know if she even finished, but she was definately a crowd favorite.
A friend of mine compared a fixed CX race to playing a really muddy game of pick-up tackle football with friends who are all better than him. It's awesome, even though your performance might not be so hot, you get beat up but but it feels great, the crowd loves you, and for the next few days you're finding mud in places you didn't even know you had.