Need of External Pull Up/Pull Down for Processor I/O pins
It depends on the other device. If the other device is very high impedance then yes, the pull down will be effective.

But it depends on the values of things. If you know the values for things then you can solve for them.

It's best not to think in terms of "current will flow in low resistance path" and rather in terms of Ohms law (and voltage/current dividers). If you have two resistances that are about the same, then the voltage between them will be about half (if they are in series) and each will get about the same amount of current (if they are in parallel). Note that even if one is a little less than the other, all of the current doesn't just take that path. They still divide it about evenly.

So to with your pulldown. If the other end has a pullup that is about the same, your node will float to about the middle. 

If you are designing a PCB then just add the pull down. You can always decide later not to populate it.