will your MTB rim take a 1" tire? there are probably some skinnies made for the tri crowd in that diameter but i'm not sure of any tire for a mtb less than 1.25. why not get a slightly wider one and pump er' up nice and hard?

as for fixed centuries, it's not as hard as you would think. i would say that if you can manage 160km on a geared bike then you'll be fine on a fixed. i don't remember feeling any more tired ridining fixed vs geared. i remember doing a century from maple to jackson's point and back last spring and me and the fixies pretty much stayed with the rest of the club. the only place where we lost them was descending some of those huge rollers going south on keele from king rd, which was near the end of the ride anyway. i guess my point is: if you want to try it, do. push yourself and don't worry about about being fast. if you're in a hurry, why are you on a bicycle (as they say)?