Hi, I also run a proxy server, but only to save my own bandwidth usage. (1 pc.)
I use squid, and have done for several years.
I really can't help you with your requirements, But I do know a few sites that do.
The first one is "linux home networking"
and no, you do not need to pay to use the info on the site ($)!
I have found this site invaluable for both mrtg, squid and iptables firewall.
If you do decide on squid, which is available for all nix's, the squid site pages....
you'll find them all here on the left-hand side of the page under documentation.
I generally use...
# Configuration Reference
# Configuration Guide - Visolve
# Configuration Examples
# Users guide
these include an api-like page, config and examples.
And yes you may achieve all you have listed, I'm nearly sure.
Cheers and regards Glenn