Non-SSD 1TB iMac, SSD Addition
I notice in the Guide for the EMC 2546 that you can replace the SSD, the guide shows the internal 3.5" HDD and then a SSD also in the system. Does this mean my non-SSD/Hybrid iMac has the slot to add a SSD myself?
No, Apple offers two drives in this system (HD & SSD) and two different ports for these drives.
The HD is a SATA-III connection but uses a custom header and Apple still uses custom HD's that have internal sensors that can be monitored to control the systems cooling (The standards group has altered the SMART standard for full time temp monitoring now) it was too late for Apple to use on these systems.
The SSD connection uses the same SSD the MacBook Pro retina models use (only for the i5 2.9 GHz and 3.1 GHz iMacs, a different unit is needed for the i7 3.4 Ghz model).
If you haven't gotten your system yet, I would strongly recommend getting one with the dual drive setup from the get go Vs getting it without the SSD thinking you can put it in later. Apple didn't make it easy to get into these systems and the SSD is on the back side of the logic board so it's a lot of work to get to.