Yes, all of the Mac mini's can hold a 2.5" SSD unit in-place of the SATA HD.
If you are thinking of replacing the optical drive then you will need a HD/SSD carrier to hold the drive.
Frankly, I would recommend replacing the HD with a SSHD (HD/SSD hybrid drive) as the better option.
If you are adding the SSD as a second drive then that works as you can then create a Fusion drive with the two drives (HD and SSD).
My home mini is setup with two - 2TB drives as a media server.
At work we use the mini's with hybrid drives. We had some older systems with the optical drive pulled and put in SSD's in place of the optical drives. We later stopped doing this and have gone with SSHD.