DNS Cluster with Multiple Slaves, Outsource Mail

Hi All. I am trying to get to grips with the network layout for a multi slave, multi dns WHM/DNSONLY setup for a new managed web cluster managed by WHMCS. The cluster is likely to consist of 6 Slave servers to begin with srv1.domain.com, srv2 and so on, plus two DNS servers in redundant locations ns1 and ns2.domain.com. We will run an out of band network for shell access to all servers in this cluster using the redundant network ports on the servers for this, which will also carry the protocols for our NMS. WHMCS will have the customer portal and DNS Management plugins installed to keep things simple for customers to manage. I haven't deployed a setup like this using WHM/DNSONLY before so need a little guidance as to address schemas people have used that works and cable configurations. We are planning to backhaul everthing back to two switches for redundancy and just channel group the NICS on the switches and trunk them on the servers if WHM can cope with this? Any guidance, experience or pointers where to get started would be great. It is our understanding so far that we can allow the DNSONLY servers to take the weight and automatically update zones on the corresponding client servers automatically. Also the cluster is likely to grow to 30+ servers. Are there any known issues with this sort of setup and platform with this many servers? Thanks in advance all!