Yes.
I use primarydomain.tld for RDNS. I have never used mail.primarydomain.tld so I don’t know if it will work or not.
It is not essential for you to host your own name servers (i.e. use Virtualmi’s nameserver) and if you manually copy over the DNS settings for a virtual server from Virtualmin’s DNS to the third party DNS service that you are using, you will be fine.
See: