I have certain dates that I can upgrade servers, however I'm about a month away from one of them. I wanted to know if I can install a new web server as Debian 8(on a multiserver setup) without causing any issues. At a later date I will upgrade the other servers to Debian 8.
Yes I have multi server setup up with 10+ servers about 50/50 ratio of Wheezy and Jessie. I have seen no issues so far.
You can mix even different OS (Centos, Ubuntu, Debian) in a multiserver, just ensure that the ISPConfig version on all nodes is the same.