It looks good to me so far. I can see how it could get too busy (bet the real blueprints of everything under NYC streets would make an engineer go crazy) but I think you could put in a fair amount of detail depending on how you present it. More is sometimes better.

On the source material...as long as you don't actually include it in your finished product I think it's probably okay. I'm basing my map on a tourist map of the city, but I don't plan to actually copy any of it. I believe this is okay but I am not a lawyer so...
M