Hi Stefan, the dual hard drive kit is compatible with any (12.5mm or thinner form factor) hard drive that the mac mini is compatible with. SATA platter drives (normal hard drives) and SSDs will both work.
Our selection of compatible drives are available here: http://www.ifixit.com/Mac-Parts/Mac-mini..., and these are some recommended options:
SSD speeds from the Momentus XT Hybrid Drive. — Currently out of stock.
SATA interface for Intel Macs / Seagate / 2.5" wide x 9.5mm tall / 750 GB Unformatted / 7200 RPM — 38 available at 129.95 each.
SATA interface for Intel Macs / Toshiba / 2.5" wide x 9.5mm tall / 1 TB Unformatted / 5400 RPM — 9 available at 129.95 each.
Add a second hard drive to your 2011 or 2012 Mac Mini — 48 available at 49.95 each.