See my responses below:
"1. for example i use
tariq@tariq.com as the simple identity, on the remote netscreen.. if i entered
sako@sako.com i still can login with the user tariq.. !!!"
How is the IKE gateway configured on the Firewall? Is it configured with IP address or as a dynamic user? If by IP address then it would not matter what email you send for IKE user ID. If the public IP address of the client matches the IP address configured on the Firewall then it uses the IP to identify IKE user and not u-FQDN.
"2. yes the both are using ipsec service, is there anyway to let them both work on the same pc !!?"
Not likely both software can co-exist on the same machine. The two software can conflict with each other on the PC causing performance issues and unpredictable behavior in the best case and blue screens in the worst case. The NS-Remote Installation Guide specifically states to uninstall any other vendor VPN client apps.
"3. its xauth and locally, can the user change it ?"
Local xauth users and passwords cannot be changed by the user. This can only be changed by a Firewall administrator logged into the Firewall via CLI or Web. This may be possible if xauth was via Radius with an Active Directory on the back end. But with local users, this is not possible.