I think you should set cron to run every minute, and then put limitations in the delivery server settings. If the server has crossed the limit, nothing will be sent even if the cron runs.
The delay between sending is so you don't open too many connections with a domain in a short span. Its not necessary but why not give a gap if the same number of mails get sent per hour. Its better to send the 500 emails slowly over the hour, rather than within the first 1 minute.
The general cap per hour will work if the cron is running every minute.
Depending on the traffic you could put many sites on one VPS, probably many more than 10.
In summary, I would
1) set cron to run every minute
2) in the delivery server settings, set an hourly quota of 450 (to give some gap in case your websites send mail)
3) in the delivery server settings, give a delay between sending of 8000000 (8 seconds).