Ah! This is a big one!
Lets start off talking about the PCIe bade SSD Apple uses. So far no one offers a 3rd party replacement SSD for this series. Frankly, I think it's going to be awhile, I would keep an eye on OWC here: iMac SSD Upgrades. With that said you might be able to find an Apple authorized service shop to order you the 1 TB spare drive but it will be very expensive! So far Apple hasn't allowed anyone to get them as spares only as a warranty swap out. But, I think they will be softening this policy soon.
Next, lets talk about why you don't see the discreet drives in the finder in a fusion drive set. This is to be expected! The OS see's the two drives as one storage space (virtual volume). To see the drives you'll need to look under the system report under 'About this Mac' There you'll see the two drives.
As to the Samsung SSD I would recommend you stick with the EVO series Vs the PRO. The cost factor is not worth it unless you are editing or creating full length movies. Here's the spec sheets for both:
FYI: There is no 450 Pro SSD option here.
OK, we've tackled the easy questions so far so what to do?
Yes, you can break the Fusion drive set so each drive (HD & SSD) are seen discreetly Vs as a virtual drive you have now. Sadly, you can't create a fusion drive set using two SSD's. So swapping out the HD for a SSD while doable won't give you the max performance you could get here. As the replacement SSD will be limited to the HD's SATA port speed: SATA III (6.0 Gb/s). I also must warn you opening this system is a bear! And if you don't use care you can damage the display assembly which would be a very expensive mistake!
Instead of altering your internal storage how about going a different direction, I would recommend going with an external Thunderbolt2 RAID drive set (RAID 0) using SSD's. Here we can achieve faster performance than the built-in SATA III port effectively driving 2 or 4 drives in parallel! 2 drives 6.0 Gb/s = 12 Gb/s theoretical or 4 drives 24 Gb/s theoretical.
Once you set it up you can then switch the startup (boot drive) to be the external drive, gaining the its speed. Sadly the internal drives are of little use at that point.
Here is what we use: OWC ThunderBay 4 mini.
OK so you're going for the internal SSD solution:
Here's the deal you first need to break the Fusion Drive set. Follow the instructions here: Mac World - How to split up a Fusion Drive
Next you'll need some tools and the tape strips to replace the ones that currently hold your display on. Follow this IFIXIT guide to letter! If you don't Opps! expensive replacement display: iMac Intel 27" Retina 5K Display Display Replacement and here's the guide to replace the strips at the very end of the job: iMac Intel 27" Retina 5K Display Adhesive Strip Replacement
OK, now the fun begins!
Follow this IFIXIT guide (jumper to Step23) to take your current HD out: iMac Intel 27" Retina 5K Display Hard Drive Replacement. Here we are swapping out the 3.5" HD for the 2.5" SSD which use the same SATA interface. We are not changing the custom PCIe blade drive.
You'll need your SSD at this point and you'll need this as well: OWC In-line Digital Thermal Sensor for Hard Drive Upgrade for 27" iMacs 2012 and Later and you'll need an adapter frame as the older drive is 3.5" and the SSD is 2.5". As you have to order the thermal sensor why don't you get this as well from OWC: OWC 2.5" to 3.5" Drive Adapter Bracket
You NEED the thermal sensor no and's If's or But's!
The System Management Computer (SMC) within your system uses sensors to manage its cooling. Once you disconnect one, the system goes into failsafe driving the fans into high speed overtime you'll burn out the fan's besides who wants to hear them.
No, don't use fan control software it really doesn't work long term. All you'll do is end up either killing your fan or your system. Trust me I've tried and I'll never do it again!