i think it might be managable if you're doing more cross country stuff and you take your time. i have heard of at least one guy who races a fixed mtb professionally. i guess he's exceptionally phenominal though. i think pedal position/clearance would be your biggest conflict. i've also seen bike videos of guys doing northshore stunts, technical downhill, trials, etc. on unicycles. those are fixed... hahaha but they are pro as well. perhaps you could setup a fixed/free setup and give it a whirl. i've never ridden a fixed mtb, but i've ridden my fixies on stuff rougher than i probably should be going over with road slicks and no brakes, but it has just been on rough dirt trails or roads, nothing single track oriented. my mtb is a ss, and i find that challenging enough; but i do more freeriding and technical downhill than cross country riding. i would think it would be hard to have a ratio that is capable of climbing, but not too low that you spin like mad with any sort of speed. a 29" mtb might be a better canidate than a 26" for clearance issues. i say go for it if you think you can manage. you're the best judge of your abilities.