The idea here is to setup a dual drive system (SSD & HD). The SSD will have your OS and apps and the second drive would be for your files (music, pics & vids). You'll likely want to compose vids & music off of the SSD then save your work over to the HD.
This way you're maxing your dollar as HD's (even a SSHD) is cheaper than an SSD.
Yes, you still could put in a SSD but you won't really leverage it as you'll want to use the still faster SSD as your boot drive (the PCIe blade SSD).
If you really want the SSD you'll need an adapter frame like this one: OWC 2.5" to 3.5" Drive Adapter Bracket and you'll still need the in-line thermal sensor i listed above.