Another note: thermal sensor is not required for SSDs on systems using OS X El Capitan/Sierra. I installed an SSD without one. No fan throttling at all.
You have a 2.5" SSD and not a blade SSD? If your iMac has a 2.5" SSD, just remove the screen, remove the left speaker for access to the left hard drive bracket, unscrew and remove the left hard drive bracket, unplug and remove the SSD and install the new one.
If however your iMac came with a blade SSD only, you can install a 3.5" HDD or 2.5" SSD, but you need a mount to fit the 2.5" SSD in the 3.5" HDD bracket, if there's even a bracket there at all. If not, find a creative way to keep in it place I guess...
You also need a specific SATA cable (see guide I linked below). Use it as a reference to see where the HDD/SSD is mounted and where the SATA cable needs to plug in (it's not a normal SATA cable). I was able to connect the it without removing the logic board, but it's not easy as the logic board is installed backside up. It needs to connect to both its on-board power connector and the on-board SATA slot:
iMac Intel 27" EMC 2546 SATA Cable Replacement
No, unfortunately. The proprietary Apple SSD has a unique slot configuration. It just simply won't fit. I've replaced the SSD of my iMac 27 5k and had to buy the SM0512G.
See the pictures below and look at the end of each. They are different.
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