No you are totally missing the point.
Do not put caps on the signal line I never said that!
Your motors are generating interference so you need to suppress this. Your chips are picking this up you want to prevent this. Your signal lines you want to minimise this.
A pull down on the signal line will lower the output impedance and so make interference more difficult to pick up.
A capacitor across the power supply connections of your motors and your arduino will stop interference on the supply. Typically you would use a 0.1uF ceramic cap.
You need a Pi filter supplying the motors, with this you need bigger caps say 47uF or larger as well as 0.1uF ceramics across them for high frequency response.
This is all simple basic everyday stuff, not rocket science.
All wiring should be as short as possible, all grounds should meet only at a common point, a so called star ground.