The extra step integration into X-Cart I'm not too familiar with unfortunately. I've performed the steps in our own website but it's a Wordpress website and integrated using that.
With regards to the various subdomains - you can see them on our website as
http://cdn1.handsonwebhosting.com up through cdn6
We then call all CSS calls from css.handsonwebhosting.com
I guess, using some creativity and programming, you could setup say 6 subdomains (cdn1, cdn2, cdn3 etc) and then making the system choose at random to pick from 1, 2 or 3 on each call. It would definitely require more overhead computer power though.
Here's an output of how the CNAME for the different subdomains would look-
http://www.digwebinterface.com/?hostnames=handsonwebhosting.com%0D%0Acdn2.handson webhosting.com%0D%0Acdn...4&nameservers=