I am un-sure if I need to use a resistor on each led.. or can I combine all negative leads and use 1 resistor for the total?
You need a resistor on each LED. If the LEDs share a resistor then the brightness of the LEDs will depend on how many you have on.
however.. I dont get a change in the reading until I am under 5v+..
Then you have not wired it up in the way you have shown, the values look fine, although a volt meter with an input impedance of 1K5 is a bit of a poor meter.