Hi! I don't have this piece, so my vote doesn't really count if you are only looking for input from owners.
However, here are my thoughts for what it's worth...
I do realize everything is 100% preference, and there are owners who have already responded saying this love this combo.
For me, I vote no. Here is why...
I think the RG GMOP pave 20 is pretty, but as a statement necklace, it's very subtle and neither here nor there.
The alternating 20 to me does not have the pop of 20 of the same colored stones such as MOP or onyx. You can't miss those "20 stone" necklaces; they pop without diamonds. Diamonds are not the end all to me, and for Alhambra, I think the beauty of Alhambra is the continuous stones that make a statement.
I know the alternating 20 has "diamonds," but they are so tiny, you can barely see them. You mostly see metal.
I tried on the WG onyx pave 20 as I thought I might get an alternating onyx pave set. I was disappointed and passed. It neither popped like a 20 onyx, nor did it have enough bling to sparkle like a diamond necklace. The diamonds were so tiny, the eye has to stare to even notice "oh, there are tiny diamonds."
However, my biggest disappointment was the "air space" created by the pave motifs. Those motifs seems to be secondary against the bold pop of the stones, so to me, and alternating looks like a metal chain necklace with a few motifs sprinkled.
I also think for the RG GMOP, the stones will have a orangey-brown undertone. This is because VCA wants to complement the RG metal which has a bit of orange-tint due to the copper in the RG. As you know, VCA varies their choice of MOP based on the metal. For YG, VCA uses MOP with a more pinky undertone, but for WG, VCA uses MOP with a cool silver undertone. The two MOP necklaces look completely different as will the RG MOP.
That said, I think the RG GMOP 20 worn with your RG Clover set will not pop enough. The necklace recesses to me, and I think it will look really mellow and subdued. It is a lot of metal, especially when worn with the Clover hoop earrings. However, that may be the look you want, and if yes, I think you will get that.
What to get then for a necklace?
You mentioned you will most likely get the Cosmos PG MOP ring (I assume it's the BTF ring). I think Cosmos with PG MOP is stunning. I've seen the earrings, and the combo is gorgeous! The MOP is pinkish as well; it is not cool gray like WG MOP. The MOP just pops and comes to life, much more so than my WG MOP.
If you are going to get the Cosmos RG MOP BTF ring, I would get a RG MOP 20. You would have to SO it, but I think it would be worth it to match the Cosmos RG MOP ring.
So... imagine now your set... Cosmos RG MOP ring (pop!) and RG MOP 20 (more pop)... two bright pieces. This would be next to your Clover bracelet and Clover earrings, which are two softer metal pieces with a touch of sparkle. I think this set will come alive much more so than a RG GMOP 20. Plus, I am not a fan of wearing the Cosmos RG MOP ring, the VA RG GMOP 20, and RG metal Clover pieces. That's three themes going on, and I think the MOPs tie the Cosmos and 20 together better. That's just me of course.
You might be poo-pooing this RG MOP 20 idea as you already have a WG MOP 20. I don't think it's repetitive. I have multiple pearl necklaces, and I wear them singly or layer them. You could do the same with two MOP necklaces, wear both and your theme is MOP with mixed metals. I think that would be a cool look. Side note... I have actually thought about getting a second WG MOP 20 to layer as I think it will be a very chic look.
If not, and you're still poo-pooing the RG MOP 20 idea...
Another option if you like alternating is to get the RG MOP and guilloche alternating. I think the launch is for Korea only, but it will eventually come to the US. Or, since you are such a VVIP, your SA may be able to get one for you from Korea when it launches.
The shiny metal in the guilloche will be a nice complement to the bezel edge of the Cosmos ring which is also shiny.
By the way, for the Cosmos BTF ring, I looked at the oynx diamond version, and I did not like the bezel in WG against the onyx. However, I saw the RG MOP version, and the RG bezel is stunning against the MOP. Your ring will be gorgeous.
For me, I would stay with a RG MOP theme, especially to match your ring. It also brightens up your set in my opinion.
Just my two cents for what it's worth. Good luck to you and congrats on your future pieces!