I'm going to offer a dissenting opinion here and say you should go straight to riding fixed. If you get used to riding with the freewheel, you will be a LOT more likely to accidentally try and coast while riding fixed later on. If you go straight into it, you're going to have to learn right away how it works when you never stop pedaling, by feeling that kick at a lower, safer speed the first time you try to coast. As long as you leave the normal brakes on the bike too, it's perfectly safe, and I think an overall better plan.

Also, welcome to SF, come out to a social ride once you're comfortable on the bike!