Hi jdc -
If you have a main script (the one that looks at the linkspool and runs spider processes), keeping track of the number of spiders is easy. Just increment a counter every time a spider is called, and when your counter variable reaches 5, you can sleep the script for a period of time and then check again.
To kill the process, check out this thread:
http://www.phpdig.net/showthread.php...&highlight=PID
But instead of using a CRON job, you could use exec() or system() commands through PHP.