Down here you ARE allowed to take your mt. bike on the track as long as you have your NBL or ABA license. I believe they do want you to zip tie your quick releases and you still need to use a pad set. There was even discussion about having a seperate class for mt. bikes!
Either way, have fun!
Gearing I'd say big ring up front like 42 and middle cog about 16 or 17 in the back! My BMX cruiser is a 41:16 but that's 24" wheels. So basically play around till you find a good gear.
__________________
"Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming, "WOW, What a Ride!" - unknown
"Your Bike Sucks" - Sky Yaeger