With sni, you can run as many ssl sites on one ip and port 443 as you like in apache. Sni is supported by ispconfig out of the box, the above described setup is not required then. All current browsers support sni, only the old internet explorer in windows xp does not support it.
To use sni, ensure that sni is enabled under system > server config > web. Then you can enable ssl on every site even if they use the same ip address without doing any manual configuration.