Don't worry about some loudmouth on a message board telling you you need a brake on the rear of your fixed mtb. From someone who's raced both run of the mill mtb races and one of the 24hr variety fixed, lets just say its up to you. And that I don't personally know any fixed offroaders running a rear brake.

We're all just fine, and I guarantee more experienced than this dutret dude. Give it a rest, we're not harming your first-born by not letting you inspect our bikes for an approved build first.

As for how to convert your Monocog, I'm a big fan of the bolt on fixed cogs that fit a 6-bolt disc mount. Riding fixed offroad you are using much lower gears than on the street, and thus much more torque is on the fixed mount (be it threads or bolts). The bolt on versions seem to hold up a bit better than traditional threaded mountings.

...back to your regularly scheduled soapbox derby...