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.