Just to check - the 27" LED Cinema display has a captive lead that splits into three connectors - MagSafe, Displayport and USB, and just USB connectors on the back. That won't work with the TB2 adapter. The Thunderbolt display looks identical from the front, but only has two plugs - MagSafe and Thunderbolt - and extra ports on the back - that should work with the TB2 adapter.
No Apple adapter - and not a very wide choice of USB-C to Mini DisplayPort dongles (well, lots of very similar looking ones under various brands). If you can't find one known to work with iMac Pro, make sure that it is known to work with the 2017 iMac/MBP - there were some issues with older adapters on these machines which are likely to also apply to iMP. A USB-C to full-size DisplayPort cable with a DP-to-MiniDP dongle should work.
May be a good time to think about whether you also need more/accessible USB and other ports - if so, a hub/dock with DisplayPort might be a contender (there are a few with MiniDP, otherwise its a DP-to-miniDP dongle) - since its only a 1440p display, a USB-C dock would do.