QFMFT. Take it from a roadie and fixed rider. Fixed gear is great endurance training as well.

Get a front brake because the rear of a muni bus or car at 20mph+ is not comfortable at all and there will be no such thing as a spin out for you. You'll need a larger cog as well, the hills of SF are unforgiving. A good ratio would be 49/21; sounds wimpy but trust me any smaller cog and the hills will claim you especially if you're new to fixed gear. Don't get carried away by the guys on the mash and macaframa movies. And seriously? While you're riding you're actually going to stop, remove your greasy chain to flip your hub and replace for a different size cog?

Do not use tubular tires. They're much more expensive to repair* (if even possible sometimes)/ replace/ install. Get a street oriented tire such as a gatorskin.