Steppers are not constant current devices. In fact they will almost never run at 6A. The current number is a average over time. The instantaneous back EMF wave will make a much larger current spike depending on load but at any RPM over about 100 the current will be much less.
The wire is large enough but the thinner wire will create more RFI than thicker wire because of the power dissipated in the wire. I would guess the wire will get a bit warm when being used.