The advantages of the G-rex, are that it can output pulses much smoother and faster, 4,000,000 steps per second vs 45,000 max from a parallel port. You also get many more in puts and outputs.
The G101 is for developers, it's not complete. The G100 is for end users.
The drives just get the step and direction signals from the G100. Any step/direction drives would be connected the same way.
Mach4 is no more. I'd expect at least a few more months for a fully functional G100 running in Mach3.
Setting up a G100 in Mach3 is pretty much the same as a parallel port.