Can new iMac output video to current generation Thunderbolt display as secondary monitor? I have a 27" iMac hooked up to 27" Thunderbolt display, providing me with a standard dual monitor configuration. Both the iMac and the secondary monitor were purchased on the same day in early 2012.

Apple recently released a 5k iMac which I plan to purchase. I'd like to swap out my old iMac for this new one, and hook up this new iMac to my existing Thunderbolt monitor so as to retain the dual monitor configuration.

Will this work? I realize that the secondary Thunderbolt monitor will not support the new 5k video format, seeing as how it was made back in 2012. But will it at least run properly at its own native max resolution? Are there any downsides (e.g. eyestrain, etc.) with having a dual monitor configuration wherein one monitor has a significantly higher resolution that the other?
Your dual monitor setup will still work fine, and they will both stay at their native resolution.

The only issue is that you might have the borders will not match up, since 5k is (probably) a lot more than what you had before, so it will be a lot larger. When mousing from your old monitor to your new monitor, you will have no problems, but when mousing back you might hit a boundary on the top or bottom. You can configure where the boundary is, so it shouldn't be a huge problem.