Could I use my blue tooth keyboard from my iMac desktop computer (mid2012) with my new 12.9 iPad Pro (2018)?

Well, all you have to do is give it a try. I'm using a bluetooth keyboard from my 2015 iMac on a 2013 iPad mini and it works just fine. Just go to the iPad's Settings app and choose bluetooth, turn it to "on" and then turn the keyboard on. If it pairs you'll see "connected" in the Magic Keyboard window under Devices.

it should work with all bluetooth keyboards in the world, bluetooth has what they call profiles and if a bluetooth keyboard support the profile it's one size fit all I've used the iMac keyboards and windows layout keyboards from microsoft and other generic keyboards, if put into pairing mode and not having an active connection to another device at the time they pair right up. this is not the same with bluetooth mice and iOS though because the operating system has not cursor

if one experience issues one can forget the keyboard and repair it after making sure it had not connected to another device I had a long session trying to find out why it did not work only to find out the keyboard had connected to my mac I typically have 4 mice and 2 - 3 keyboard connected to it by various means

Hmmm. I have only used my magic keyboard with my iPad occasionally, such as whenever I am asked to input my password for some reason and it has always paired up for me when I needed it. I HATE to use the touchscreen keyboard for anything :-) Perhaps some other devices are interfering with it? Maybe you should make sure it is disconnected from other devices first. In my case I don't have anything else connected to it.