Pull-down resistor confusion
The pulldown is just like the pullup. In both cases we are concerned with the situation with the switch open, since the resistor doesn't enter into it otherwise (except to prevent shorting the supply, as you say). 

In the case of the pullup you have an input which is desired to be (say) at 2.0V and is sinking a current of 20uA. The current flows through the resistor. So the maximum value is (5V-2V)/0.02mA = 150K. 

In the case of the pulldown you have an input which is desired to be (say) at 800mV and is sourcing a current of 400uA. That current flows through the resistor. So by Ohm's law the maximum resistor is 0.8V/0.4mA = 2K. 







simulate this circuit &ndash; Schematic created using CircuitLab

In reality we would likely use 2.4V and 0.4V for the voltages to give 400mV noise immunity (LSTTL levels)- rather than zero noise immunity- in a real design, so the resistors would be 130K/1K and 130K is too high impedance and invites EMI to enter so more likely it would be something like 10K or 4.7K and 1K.