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:
SATA 6.0 Gb/s interface for Intel Macs / SSD Hybrid, 5400 RPM / 2.5" wide x 9.5mm tall / 500 GB Unformatted — 15 available at 109.95 each.
SATA 3.0 Gb/s interface for Intel Macs / 2.5" wide x 9mm tall / 1 TB Unformatted. — 24 available at 109.95 each.
Add a second hard drive to your 2011 or 2012 Mac Mini. Kit includes: SATA Hard Drive Cable, Mounting Grommets, Mounting Screws, Logic Board Removal Tool, 26 Bit Driver Kit, and ESD-Safe Spudger. Compatible with 2.3, 2.5, or 2.7 GHz 2011 Mac Minis (Model A — 4 available at 29.95 each.