Alienware 15 R3: Questions About SSD/HDD Installed

So, I got my first Alienware laptop almost 2 weeks ago, it's not new but it is lightly used, I've wanted one for years since before Dell bought them -- wish that would have never happened -- and I was looking some information up and came across the following site:

https://www.laptopmain.com/alienware-15-r3-disassembly-and-ram-hdd-ssd-upgrade-options/

Now, this laptop has a 128GB M.2 SSD in it as well as a 1TB HDD.

As you can see from the following image, there's an SSD-1 slot, but what can actually fit in there:

I know I can fit an M.2 in the SSD-2 & SSD-3 slots, but like I asked before, what goes in SSD-1?

Can I replace the M.2 with an NVMe version, I believe I've come across things saying it can be done, but I want a "yea" or "nay" from the community.  Also, what is the biggest size HDD that can be put in here?  I know with the 2012 ROG that I bought used 2yrs ago, it would only support a maximum of 2TB, since this laptop is newer, can something bigger be installed?