Oh boy.
You should post this in ss/fg 1st of all and then take the flaming as welcoming.
You could THEORETICALLY run a flip flop and thread a 5-7 speed FREEWHEEL onto it but you would have to have a frame with the proper spacing and if you were running fixed on one side and a freewheel on the other side you would have a derailler anyways so what would be the point? Not to mention... when you ran it fixed your chainline would be a disaster (most likely)
What I would suggest...
1. Goto Sheldon Browns website and read read read.
2. Get yourself some cheap POS with horizontal or semi horizontal drops.
3. Get a flip flop hub and run it fixed on one side and single speed on the other side. You can gear up by 1 or 2 teeth on the single speed side so that if you REALLY can't get up a hill or something you can flip it and go.
If you were going to build a geared bike to commute... I would just buy some cheap pos (used not new... a used POS is most likely an actual "real" bike instead of some wal-mart crap) and use that as is.
Welcome...
Don't leave if the fixed gear / single speed guys trash you.