Steppers take full current whether moving or holding position (unless explicitly switched off, which means they can be turned easily and you lose position).
So 7 steppes @ 1.7A = 11.9A. Times 1.5 for safety (power supplies often can not supply the full power for long, they'll get really hot) makes for an 18A supply.
The rest doesn't need much power, and can piggyback on the main supply. Do isolate your control electronics properly from the noise of the motors, particularly the power spikes from switching on/off the coils.
[EDIT: see replies below; this is a gross overestimation of the power supply current required by the steppers as it's regulated by the stepper controller]