On each server, you will have to install the php packages (except the mod_php package, which is only needed if apache is installed too), as ISPConfig requires them for local processing. Then install only the services that you want on the server. The server can be divided in the following parts:
1) Mailserver: Install the postfix, amavisd and clamav packages.
2) DNS Server: Install mydns
3) Webserver: Install apache, vlogger and pureftpd packages
4) DB server: Not really useful yet, as the current beta lacks support for user databases.