I've currently got both IIS and Apache 2 running on my server on different IP addresses - works perfectly!
To get IIS stop hogging all the IP addresses on your server, make sure you explicitly specify the IP address for all your IIS sites then see this article:
http://support.microsoft.com/default.aspx?...Ben-us%3B813368
If you want the support tools, a Rackshack engineer will put them on your server if you submit a trouble ticket, or you can PM me and I'll send them to you (10MB).
And remember, Rackshack will give you the extra IP address for free if you haven't used more than eight yet.