How can I match the firmware between the two when the latest firmware

Bottom Line (according to Netgear tech "support"):  The ONLY satellite that connects with the RBR760 (router) is the RBS760. In other words, the RBS750 is incompatible with the RBR760. Furthermore, Netgear does NOT currently sell individual (separate) RBS760 satellites.

Blow-by-blow tech "support" experience (88 wasted minutes that I'll never get back):

Netgear customer "support" continues to be anything but that. I was reminded during my numerous calls and transfers as to the reason why I am still NOT a fanboy of Netgear.

1. I spoke with five individuals. All were non-native English speakers who were difficult to understand, especially since all were providing "support" from environments in which there lots of LOUD background noise, with women and children talking/yelling, as well as lots of birds/animals, and strange music.
2. The total time that I spent on the phone was 88 minutes.
3. The first tech support lady that I spoke with initially told me that the new RBR760 (router) and older RBS750 (satellite) are incompatible, but then spent the next 15 minutes confirming it with her tech up-chain spiritual master. 
4. When I asked if I could purchase a single RBS760 satellite, I was told that I could do so but that I would need to be transferred to "Sales". Following the transfer, and brief conversation, the sales lady told me that she was actually in the "pre-sales" department (and could not help me) - and that I needed to contact "Sales" directly. She could not transfer me, but provided me with a toll -free number to the sales department.
5. When I called the new number and worked my way through the prompts, I eventually spoke with somebody else in tech support. This person then transferred me to "Sales".
6. When the sales person finally answered the line, she told me that the RBS760 was unavailable individually and that I would have to purchase another router/satellite combo set to get another satellite. She told me to call a retail store to attempt to locate an individual RBS760 (BTW, locating one is impossible via anything other than EBAY - based on my numerous Google searches).
7. After hanging up with the sales lady, I re-contacted tech support to find out how many satellites an RBR760 (router) can handle. This tech support dude was very direct, almost to the point of being rude. He recommended that I connect no more than 4 satellites to the RBR760 (and attempted to convince me that a max of 3 would actually be best). Furthermore, when I mentioned the incompatibility problem between the RBR760 and the RBS750, he told me that the previous tech support info that I received was wrong - and that I COULD in fact connect the RBS750 to the RBR760. We spent the next 58 minutes attempting to do just that - with NO success. He then took my phone number, promised to talk to his higher-up ORBI guru about the matter, and return my call. If he calls back, I plan to book a plane to Vegas and put all of my life's savings on red - after which I will use a portion of my winnings to hire a full-time tech support wizard to deal with my wireless network issues for the remainder of my life. One thing I won’t be doing with my prize money is spending any of it on additional Orbi products.

Fortunately, I purchased my Orbis from Costco. Thus, I can return them at any time - and likely will do so. In fact, if it weren't for Costco's fantastic return policy - and the fact that they carry Orbis - I would have never purchased another Netgear product. This said, I have looked at Orbi alternatives and all seem to have their issues.

As for the question re why I need so many Orbi satellites:

I have a couple of VPN access points that must be connected directly to ethernet ports (no wireless) in areas of my house in which there are no ethernet jacks. Therefore, I can simply place an Orbi satellite in each area and connect the VPN access points directly to the back of each satellite. I then can plug my computers directly into the ethernet ports on the access points.

Thanks much for all of your ideas and assistance - and best of luck navigating the rat hole that is Netgear!