I build a new suexec, because it is easier to add a new web account in ispconfig. and it make it easier to edit the script as root.
but you can also set the immutable-bit in /root/ispconfig/scripts/lib/config.lib.php. then ISPC. set it automatically after copy the php-fcgi-starter into the user folder.
all web accounts with php have a .php-fcgi-starter script. there can you set some paramaters for fcgi/php (like a php.ini).
in the vhost for this webs ispconfig create a ScriptAlias to the startscript.
ScriptAlias /php/ /var/www/web1/
and in the fastcgi config define the php start script with this parameter:
Action php-fastcgi /php/.php-fcgi-starter
so you start a fcgi-server for the different vhosts.
fastcgi start it for web1 in /var/www/web1/.php-fcgi-starter
and web2 in /var/www/web2/.php-fcgi-starter