I got this from an SA, so here is a guide:
2 links out = 7.0"
4 links out = 6.5"
6 links out = 6.0"
8 links out = 5 3/4"
Based on what you measure for 4 links out, you measured it at 6 3/4". Assuming the same ratio that removal of 2, 4, or 6 links shortens it by half an inch, in your measurement if you remove 6 links, you would end up with a bracelet that is 6 1/4".
That might be a bit tight. Even though it doesn't move to scratch your other bracelets, you may want to think if you want to deal with struggling to put something on that tight. It would annoy me personally, and I would probably just not wear it.
May I suggest this? Since you already have a MOP bracelet that you rarely wear, how about taking it to VCA and have them take out two more links, for a total of 6 links out. It should be free of charge. See how you like the way it fits. If you like, get the a 5 motif oynx. If you do not like, skip it and get the 10 motif.
Stacking bangles (Cartier) with bracelets (VCA) is tough if you are sensitive about scratching or twisting. They wil get twisted unless your bracelet is almost skin tight and doesn't move. Maybe skip this look and just get the 10 motif.
I definitely would shorten you MOP to test out the length before diving into another "bracelet" that you won't wear.