Can the 2019 27" iMac be setup in a dual monitor configuration with the iMac being 5K and my second monitor being 2560x1440? I realize I need a TB to DP (or HDMI) cable for this. Anything else I need to know?
Yes, it can. The 2019 iMac has two thunderbolt 3 (USB-C) connections on the back. You'll need an adaptor to use it with an HDMI or DP monitor, but other than that your plan should work fine.
That's right: the 2019 iMac will take 1 5K display or two 4K displays in addition to the internal 5K display. 2550 x 1440 is 1/4 the resolution of a 5K display (or if using the 5K as a pixel doubled Retina display the same "size" but lower resolution).
That's right: the 2019 iMac will take 1 5K display or two 4K displays in addition to the internal 5K display. 2550 x 1440 is 1/4 the resolution of a 5K display (or if using the 5K as a pixel doubled Retina display the same "size" but lower resolution).