In my company's zone file, we have NS records that refer to this third party hosting site, and we have an 'A' record set up that associates the my.domain.com domain to the IP address of a load balancer that balances traffic coming into the site between two web servers, all in a DMZ. We also have an MX record that refers the my.domain.com to a mail server host, also in the DMZ. Finally, we have PTR records that refer back to the load balancer and mail server hosts.
All this is set up correctly, everything works, and all DNS checks pass.

I need to add a subdomain, call it sub.my.domain.com, and I would like to associate the subdomain to the same load balancer if possible. What is the best way to do this without disrupting the incoming traffic to the primary domain or
the flow of incoming SMTP traffic to the mail server? Is it best in this situation to use a CNAME to map the subdomain, or to use a whole new 'A' record? If I create a new 'A' record, I would not want to add a new PTR record for the sub.my.domain.com as the IP address would be the same as the PTR that refers back to my.domain.com. This I
believe, would cause DNS checks on my mail server to fail, since there would be two hostnames associated with the same IP address.