If this is a big, well known firm that does landscaping, wall building, etc., odds are really good they've got someone who knows enough to get a 5.5' tall retaining wall right or they'd have been sued out of existence long ago. They've probably got an engineer with the proper licenses on staff (or retainer) to do and sign off on the work in order to get permits approved and to meet their insurance requirements.
If, on the other hand, it's "Bob's Handyman Service", there's no telling whether or not they have knowledge of, or respect for, the engineering and building codes that goes into making a safe and secure retaining wall that will last.
The remainder of this answer presumes you're using a contractor from the first category, not the second.
I would be sure that they know you want the railing on top of the wall cap and the kind of railing you want. Have them design it into the structure so that it will be properly supported. I'm not sure if you're planning on building the railing yourself or having them build it for you. If you're going to DIY after they're done, they may be able to build in posts for you or places for you to put the posts so that they'll be properly supported and the wall will be designed for the stresses that may happen there.
If you try to add it on afterwards, you may run into issues with securing the railing to the top of the wall and/or compromise the strength/stability of the wall leading to a potential collapse. Also, any work you do afterwards will likely open you up to the liability of a wall failure instead of leaving it with the company.
Also, you might consider making two terraced steps (as noted in Steve Wellen's comment to avoid needing any sort of railing/fencing at all. Where I am, a deck that's less than 30" above the ground doesn't need a railing (though ours looked odd without one, so we added one). The requirements for a landscape terrace may be similar where you are. If you add a second terrace so that neither wall is more than the "no railing required" height, you could get away without a railing and it may end up looking better. I'd assume that the cable railing is to minimize the impact on the view, so no railing would be even less impact.
You don't need a huge area for the lower terrace - maybe only one foot wide so that it doesn't encroach any more than necessary on the surface area of the top terrace. It would consist of the wall itself, then the exposed area between the lower wall and upper wall could be covered with gravel so you don't have to deal with dropping the lawn mower down there to take care of a narrow strip of grass. Perennial plantings (with small roots, so as to not destroy the walls) might be an option, too.
Two shorter walls may be less expensive to build and may (mostly? completely?) offset the cost of the single taller wall and railing system. These would be questions to address with your builder.