1) Our VPSes utilize CentOS 4.6
2) At this time we can only offer CentOS 4.6.
3) Yes, there is just a $25 charge for script installation.
4) It would be difficult if troubleshooting is necessary, likely with each ticket you would just need to note that you are using a custom mail setup, and the techs should be able to go from there without much trouble.
5) We will extend the 50PROMO code if an upgrade is made within the first 30 days, after that time however the upgrade would be at normal pricing.
6) We do not have a list online that I know of. Basically we will cover any problems that may arise, troubleshooting issues, etc., as well we ensure that service monitoring is enabled locally on your machine, and configure the VPS entirely at the start to be ready for hosting as quickly as possible.
7) The footprint of cPanel itself has been reduced with the new VPS-friendly releases, with Red5 running as well, I'd say around 100-150MB would be used in these base OS/system processes. If stats processes are running that could eat another 25-100MB depending on the size of the stats logs running at the time. Leaving an average of 350-400MB for your apps/sites to run comfortably within.
If you have any further questions please let us know.