Rules vary from place to place. Around here, you could use disc brakes.
Courses also vary, but they're generally mostly flat. You may have trouble with some hills, but with the 33-20 you'd be spinning out all over the place. You could buy a couple of smaller cogs (16T and 17T or 18T) and get to the race early enough to change it once you see the course. You can get Formula cogs for $11 shipped from thebikesmiths.com.