Matching Memory Modules, on Zbook-15 G6

[nogginz]

About installing more memory in my Zbook 15 G6.
I have a the model 8LK80UTR#ABA, shipped with 16gb as the 'primary memory module' for this Zbook, found in slots 1 & 2. Accessing these Primary mods requires extracting many other parts, a long, difficult procedure. Not going there, could easily cut something. Instead I'm upgrading the 'Customer Accessible' modules 3 & 4, easily accessible from the back.
So really 2 issues here.
A) Matching the basic memory type except for size, including CAS Latency.
B) Must memory sizes in 3 & 4 match whats in 1 & 2, for dual-channel to work?

Re A), Memory type = I plan on purchasing this memory pair =>
F4-2666C19D-64GRS by G.SKILL International Enterprise Co., Ltd.
I used some benchmark apps, e.g. CPU-Z, which said the memory now in slot-1 is
M471A2K43CB1-CTD(16 GB) > Samsung Semiconductor USA
This dual-channel memory has a CAS Latency of 19, matching the G.Skill.
But unsure about other details, like rank, etc.
If I install the G.Skill as 2x32 in slots 3&4, will the laptop have performance issues ?
Seems this Zbook does not allow adjusting memory in the BIOS.

Re B), Matching sizes for dual-channel behavior = As one forum poster previously noted, HPs documentation on matching memory is kinda murky.
(Solved: Memory Module - matching sizes - HP Support Community - 7579742
I'm re-asking his unanswered ending Q here. The Quick-Specs on this box say,
"2 Maximized dual-channel performance requires SODIMMs of the same size and speed in both memory channels."
This guy asked, "What slot # pairs considered "both" channels?
1 & 2 ? (the pair under keyboard). 3 & 4 ? 
Is it correct that 1 & 2 must match , and 3 & 4 must match, but 1-2 and 3-4 can have different sizes? "
Ditto here. Thanks in advance.