On the ram below the tested speed is 3600 and to get that speed the voltage required is 1.35
The SPD speed is 2133 and the voltage at that speed is 1.2
F4-3600C16D-32GTZRC-G.SKILL International Enterprise Co., Ltd.
and of course that one is
Featuring a completely exposed light bar with vibrant RGB LEDs,
On the ram below the SPD speed is 2133and tested speed 2666 both of which only require 1.2v
F4-2666C15D-32GVR-G.SKILL International Enterprise Co., Ltd.
SPD - serial presence detect is what the ram will run at by default without any manual OC or XMP - extreme memory profile being set
as you can see on the links above
Intel XMP 2.0 (Extreme Memory Profile) Ready
HOWEVER as indicated on the link for the 3600
Rated XMP frequency & stability depends on MB & CPU capability.
and to answer your question regarding suitability we need to know what processor you are going to use on that board
As you can see from the board spec on the Asus site
above 2666 is overclocked so it depends on your intentions
The safest bet is to refer to the mentioned QVL list for ram on the Asus site
4 x DIMM, Max. 64GB, DDR4 4266(O.C.)/4133(O.C.)/4000(O.C.)/3866(O.C.)/3733(O.C.)/3600(O.C.)/3466(O.C.)/3400(O.C.)/3333(O.C.)/3300(O.C.)/3200(O.C.)/3000(O.C.)/2800(O.C.)/2666/2400/2133 MHz Non-ECC, Un-buffered Memory
Dual Channel Memory Architecture
Supports Intel® Extreme Memory Profile (XMP)
** Refer to www.asus.com for the Memory QVL (Qualified Vendors Lists).
PRIME Z390-P | Motherboards | ASUS USA
and the memory QVL list
PRIME_Z390-P_DRAM_QVL_20191018.pdf (asus.com)
YOU do NOT have to use ram from the QVL list, but doing so ensures it will be OK depending as I said on the processor
On the QVL list you will see
The stability and compatibility of these memory modules depend on the CPU’s capabilities and other installed devices
So in summary check your proposed ram against the list and check using the ram site at GSkill for your board
RAM Configurator-G.SKILL International Enterprise Co., Ltd.
So regarding the specific question
1.
We need to know which processor
2. It depends on what ram speed you are aiming for as one will only go 2666 and the other potentially with 1.35v - 3600