Yes, they will get to large.

Much too large.

Do you think I could just do two of them spaced 10’ apart?

You need something low-growing in front of the arborvitae. Anything that gets as tall and wide as Shasta will smother the arborvitae, blocking it's light.

Have you accounted for how wide the arborvitae will get?

You really want to plant for the shrubs’ mature sizes so that the plants aren’t interfering with each other’s growing or getting into the path or growing against the house. We don’t know what your arborvitae cultivar is, and while a few cultivars may only get 4’ wide, many arborvitae will get too large the space even without the Viburnums. Some cultivars of arborvitae can get to 15’-20’ wide over time, and I have some semidwarf ones that are more than 6’ wide after 10years. Planting fewer Viburnums won’t help because they are large enough to fill the space shown without the arborvitae Since they get 9’-12’ wide, and you want at least a foot on each side to keep them out of the path and not against the house.

Missouri Botanical Garden is my go to place for accurate info on plant sizes. Here is their page on Shasta

http://www.missouribotanicalgarden.org/PlantFinder/PlantFinderDetails.aspx?kempercode=x760

and here is their PlantFinder main page

http://www.missouribotanicalgarden.org/plantfinder/plantfindersearch.aspx

Or you can just type into your search engine something like “MOBOT Sasta Viburnum” to get info on a specific plant if they grow it.

you could plant both the arborvitae and the Viburnum in a wider part of the bed (perhaps where it gets over 20’ deep, but not in the spot pictured unless you plan to rip it all out in a very few years.

In addition to being a rather massive shrub once it is in its prime, Shasta is really not a plant that you want (or need) multiples of. It is best displayed as a single specimen as it is stunning when in bloom and having multiples will somehow distract or weaken/reduce the visual impact.

If you have room, by all means plant one. But just one !! And make sure you have sufficient room to allow for its full development. If in front of a standard arborvitae hedge, I would allow at least a 20' deep bed to accommodate both the arbs and the viburnum.

I can see having this as a specimen in several places if you have a large yard. It has spring flowers and gorgeous red fall color if in good sun. If you have a pollinator (an other related Viburnum but not another Shasta which is genetically identical) you may get berries which would have nice color as well.

The conifers are going to get much bigger than they are now also. And since the front of the bed is curved you should not be filling it with plants in straight lines anyway.