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While i'd prefer it the other way around(Z controller + Z750), there should be no issue using the 100 watt brick and the 75 watt controller. While I have not tested this setup, I expect that the controller will not burn out unless you are running an AA diesel set and a lot(8-10) of lighted passenger cars. single diesels pulling freight cars will not be a problem.

No problem as long as you pay attention to the capacity of the controller.

That is printed on the end of the controller next to the terminals. 

The Z500 says no more than 4 amps at the input plug, I have no idea what the 750's and 1000's are capable of yet. However, the Power Bricks are useful for the TIU in DCS.

All the controllers are the same.

The Z-750 says 4 amps; The Z-1000 says 6 amps. Use an ammeter  to determine the load and multiple feeders on the track.

I just bought a Z1000, and noticed that it operates nearly identically to the Lionel CW-80.  Apparently the sine wave 'chopping' must be different if the Z causes no problems with certain engines, but it has the same 'buzzing motors and constant lighting' effects as the CW.

Just found that interesting.