OnSugarMountain,
The short story is that all cards are compatible as long as the power supply can feed the card with enough power. 
All companies that certify things, in this case a graphic card, stops to certify new generation cards for a previous generation workstation. Or, they have decided to only certify a handful of cards for a specific workstation since it is enough for the user base of that specific workstation. It never means that Card X wont work in workstation Y. Graphic cards are pretty generic comparing to let´s say processors that requiers specific motherboards/processor sockets, specific genration of the motherboard, etc.
Quadro vs GTX:
If you use software that specifically require a Quadro card, go for the Quadro card. If you use applications such as Adobe After Effects, Adobe Premiere Pro go for the GTX card unless you need the 10-bit output that a Quadro card can provide. Or, go with the GTX and get the 10-bit output via a video card from Blackmagic Design.
I use a Nvidia GTX Titan X Pascal in a Z440 without any issues.