Why are wireless signal and noise measured in negative dBm?
Because dBm represents the loss of power as the wave travels,if positive it can represent the gain in power. dBm is apart of a formula use to calculate the value of the signal sent over copper,optical and wireless media.

Should noise and signal be higher or lower? How do they effect one another?
You definitely want the signal to be higher than noise,because noise weakens the signal.

What sort of levels should be expected for a home environment? Commercial? This depends, noise can originate from natural as well as technological sources and is added to data signals in communication systems. Hope this helps