The input DC voltage is coming from rectifying the mains voltage (127 Vrms), which gives me about 178 DC Volts to play with.
With full-bridge, you get about 127V DC equivalent, and the peak at about 178V, considering possible max loading.
My question is if I need to include a buck converter to supply the correct voltages for the lower voltage rated motors, or if I could get away with using the PWM capability to drive them without exceeding their power rating.
In short, you do not need a buck converter, normally. PWM is simpler, easier to control, more energy efficient, cost effective.
Snake lags (Redundant):
DC Linear motor model is a resistor in series with a Voltage source (BEMF). Power rating is from the design factors of the motor, electrical and mechanical. In order to meet the mechanical spec (speed, torque, operating temperature, etc.), electrical parts are designed to be capable of driving the mechanism sufficiently, both with margin. Most of the time, dielectric insulation capability far exceeds the voltage spec. What can break with the voltage is to heat up the internal impedance (and junctions) by the current flowing through (thus the power).
I have used direct bridge rectified 120V line to control 68V DC motor with PWM, decades ago. The "thumper" is still regarded as professional, not gonna break easily. What prevents motor damage is the control circuitry.