Ridden fixed for over 15 years, commuting and racing on the track.
Track racing is about power and cadence - commuting is about being simple and fluid.

For commuting, two things make life good: brakes and a low gear. I have two brakes, but a single front is ok if you aren't in a hilly area.

Putting on a low gear is also good - anything in the 65-79 inch range is fine.

These two things combined makes it easy on the knees, both on the torque required to get going from a stop as well as slowing down.

Fit wise you may want to run your saddle a tiny bit higher - no more than 10mm - just to keep you from bouncing on the saddle too much. But not too much that you really reach at the bottom of the stroke.