Answer to "Replacing the harddrive with 2 drives"Put boot drive (obviously SSD EVO) to optibay position. Than you will get sata II speed insteads of SATA I speed.
Answer to "Which SSD disk is compatible with Mac Mini Mid 2010"Hi, I have installed SSD Samsung EVO 860 250 gb to my Mac mini 2010 (NVidia MCP89 AHCI) and also installed 7200 rpm HDD drive to optibay, and got an issue: SSD works only on SATA I speed (1,5 Gigabit). After some investigation, I swapped them: SSD connected to optibay, and HDD - to hdd interface. VOILA! 3 Gbs both. Hope, it will help to some body. Old native 5400 HDD gave 40 mbs speed, new 7200 HDD gave 140 mbs speed and SDD gave 260 mbs speed. OS - El Capitan.