The E-Stop signal to J14 of the PMDX-424 is low voltage and low current
and 24 AWG is fine from this respect.
You may want to use a heavier wire simply because it is easier to strip
without losing strands.
The best E-Stop setup is to have two normally closed poles on the E-Stop
switch. One pole goes to the interface board to inform the control what
has happened. The other pole should control a power contactor that will
remove power from any dangerous part of the machine in case the control
does not respond correctly. This includes things such as the spindle, or in
some cases the servo amps.