IMac – Possible to use iMac Pro as a monitor for a variety of devices

I have not seen this. Two things I have tried that help me, however, are:

Pin windows that you want on the side monitor to all spaces

I've often treated my second monitor as more a static workspace that doesn't change no matter what space I'm in. You can do this by

System Preferences --> Expose and Spaces --> Spaces -->

Click "+" under Application Assignments, and add the applications you want to stay on the second monitor to "Every Space".

Run a virtual machine in the second window

Using VMWare I sometimes run Windows or Lion on the second monitor in full screen. Add that virtual machine to "Every Space" as described above, and then within that machine I can use spaces and have it run independently of the primary workspace.

Neither method gives you exactly what you want, but they provide a few other options you might try that will still give your working style a boost.

You can not use your MacBook Pro as a second monitor for your iMac without additional software.

ScreenRecycler claims to allow you to expand your desktop across multiple computers.

As an alternative approach, Synergy (previously open source at synergy-foss.org) claims to lets you combine multiple computers into one. It will let you use both computers from a single keyboard and mouse.