I have a Mac Mini 2011 with:
Main bay: Samsung SSD 850 EVO 1TB
Second bay: HGST HTS721010A9E630 (1TB 7200RPM steam powered disk)
It works just fine ... I have a few symlinks that point over to the second disk, which I call 'Media Disk' ... in particular I symlink '~/Music/iTunes/iTunes\ Music', '~/Music/iTunes/Mobile\ Applications' and '~Library/Application\ Support/MobileSync' over to the Media Disk as those are mostly large and not often changing files. Using the symlinks that far down keeps the media directory files on the SSD for quick access.
I've been running like this for more than a year (and for over a year with a 512GB SSD before that). The machine is running 24x7.
Actually the Media disk is starting to fill up and I'm wondering whether to change it for a FireCuda 2TB, which seems to be quite a low power device as well ... probably lower than the HGST 7200! It's a shame HGST isn't making larger 7200RPM drives but so it goes.