Shorten the chain into an appropiate ratio and reroute it outside the rear derailer, save your money and see if one speed is good enough,you'll see how nice it is not having to shift or have the drag of a derailer. If you like it, invest in a dedicated hub and crank.
The question to your answer is yes. I posted a question about a couple of weeks ago what would be the best gain ratio for a road, having estimated before I wrote the thread that I would need a 5.5 gain ratio. Interesting is that all the people that answer me, and according their experience, for road a good multi-purpose gain ratio is between 5.5 - 5.7. For a urban assault machine, you need a smaller gain ratio and according my experience something between 3.8 and 4.1. Anyhow, just do like Jerseys told you and try several combinations in your present machine (without derreuiller) until you fing a comfortable gain ratio that serves you to climb and descend beside riding on flat places. Another suggestion is that you need single speed and not fixed gear because you have to face many obstacles in your urban rides and you need that your drivetrain moves freely backwards.