How different are the power supplies from various cabs. Could I put my old Galaxian PCB into a modern (jamma) cab? Would the PS work?
It depends on what your boardset requires for voltages. Some older pre-JAMMA boardsets had weird voltage requirements. A typical modern switching power supply such as you'll find in a JAMMA cabinet only supplies +12VDC, +5VDC, and -5VDC.
Do monitors each take the same basic power?
Most monitors require 120VAC. There are some exceptions, notably the Nintendo/Sanyo or Nintendo/Sharp monitors, which require 100VAC. Nintendo cabinets in countries such as the U.S. which have the 120 VAC @ 60 HZ power standard, include a 120VAC ---> 100VAC step-down transformer (which also functions as an isolation transformer).
Most older arcade monitors require an isolation transformer.