Yes, you can combine multiple dedicated servers and clustering. The guides just explain both setup types separately but they work also in combination.
E.g. if you have 2 dedicated mail servers in your cluster, then you can choose the second server in the "server is mirror of" settings of the first server and ispconfig will start to mirror all config changes from the first to the second server. Then you just have to make sure that both servers use a shared harddrive for the /var/vmail directory (e.g. use glusterfs or nfs or a NAS or SAN) and you have a mirrored mail server.
For the dns server, I recommend to use BIND, as you can use the ispconfig mirror feature for two bind systems as well without the need to create and maintain BIND slave zones.