The decision which address to use is Google's, not yours, so the single address may be down.
But you can add youtube.com
as an address to an address list. This makes RouterOS resolve that domain name to a list of numeric IP addresses which it adds to the same address list, and update the list each time the previous DNS response expires:
/ip firewall address-list
add list=youtube-watch address=youtube.com
Now depending on what you actually want to do, you can either set
/ip firewall nat
add chain=dstnat action=dst-nat dst-address-list=youtube-watch to-addresses=126.96.36.199
to force all locally originated connection to any of the youtube's addresses through 188.8.131.52, or you may use that address-list in the routing or firewall rules where you intended to use the 184.108.40.206.
But bear in mind that Google uses the same servers for a multitude of services, so the special treatment you intend for youtube.com alone may actually affect also other Google services, like search and gmail.
Instead of writing novels, post /export hide-sensitive. Use find&replace in your favourite text editor to systematically replace all occurrences of each public IP address potentially identifying you by a distinctive pattern such as my.public.ip.1.