Each RAM slot is independent so you can place one or two memory modules in them. Each slot can also support different sized modules (i.e. 2 GB in one slot and 4 GB in another).
When using two of the SAME sized modules it is important to use a matched pair as the system will try to use inter-leaving which distributes the load across both modules (in parallel) instead of using one module and then rolling over to the next (in series).
Your system had 4 GB of 1600 MHz DDR3L SDRAM (PC3-12800) installed (two 2 GB modules). Your system can support upto 16 GB of RAM using two 8 GB modules. Or, a mix of 2 GB, 4 GB or 8 GB modules as long as the specs of the modules are compatible.
As to your other question on upgrading the HD to a SSHD that is also possible if you are running out of storage or the drive is having problems. But before you go down that path first try cleaning off some of the junk you have collected as well as clearing the log & cache files. You should try to have 1/3 of the drive free so the system has some elbow room. You also may need to defrag your drive. I would get this app: Drive Genius there are others out there this one is one of the better tools.