Q: Confusion on display - Connecting a thunderbolt Late 2013 MacPro to 27" iMac C2D 10,1

You can use certain iMacs as displays for other Macs. But your iMac does not just "become" a display. You have to invoke Target Display Mode on your iMac:

Use your iMac as a display with Target Display Mode - Apple Support

As for using your MacBook Pro as an external display: ForgetAboutIt

You can use Screen Sharing, which uses software to accomplish that. It will work for most uses, but not Video Editing.

That is answered here:

Use your iMac as a display with Target Display Mode - Apple Support

Mini DisplayPort is a subset of ThunderBolt.

Adapters pick up a subset of the ThunderBolt signals, and those can be used to drive a Mini DisplayPort Display directly, or can be converted to Full-size DisplayPort (just a signal swap, co conversion) or to another "legacy" display with an adapter.

But you cannot turn Mini DisplayPort back into ThunderBolt. Signals are missing and it does not work.