Want to block users from logging in through the proxy for multiple simultaneous sessions. (Except certain specific users)

Unfortunately, it is not possible on the ProxySG appliance to limit the amount simultaneous sessions per user.

Rather, there is a process called Attack Detection where is the ProxySG appliance will drop requests after it receives too many connections from a single IP.  However, this is based on IP address, rather than user count. Also the client IP will be blocked for a set period of time.

The KB on the link below will show how Attack Detection works:

How Attack-detection works on the ProxySG appliance