Designing a power ON-OFF switch circuit
but would it be ok to do in a mixed voltage circuit, CMOS components and dc motor driver circuit?

Simply, yes.
But I would use a high-side switch instead, to keep the circuit grounded when it's off.



simulate this circuit &ndash; Schematic created using CircuitLab

The configuration above allows you to switch the circuit on and off with a logic-hi signal. If you don't want that and if you want to simplify the circuit, you can remove M2, RG, and RGS2, then place the switch between M1's gate and ground.
A safer solution is to use back-to-back arranged MOSFETs. But the one above works as well.
A PMOS with low RDS-on and enough continuous ID rating is needed.

A side note: When using an NMOS as a switch, keep the pull-down resistor (between gate and source) low. Placing a 1M resistor is nearly nothing different than leaving the gate open. Since an NMOS is a high-input-impedance device, it can still accidentally turn on (e.g. when you touch the SPST) if a high resistor is used. A 10k-ish resistor can be used instead. Plus, it's always a good practice to use a series resistor for the gate, rather than a direct connection from the switch.