Hi Cyril,
as a matter of fact, the basic concept of MIR is to have one instance where all signals come together on one stage (look at it as a mixing console). But nothing will hinder you from using 8 VE Pro instances, as long as you have them on the same machine.
You have to start VE Pro a 5.1 configuration to use surround formats (or convert an existing VI Frame using the dedicated conversion command in the File menu). In MIR Pro you will have to select one of the preset surround output formats then, or you create your own.
Like stated in the manual, instruments which are already opened in a VE Pro project can be routed to a MIR Pro Venue by simply adding the MIR Pro plugin to a channel
That said, I doubt that you will be able to run 100 individual instruments (a.k.a. "Icons" in MIR) in surround on one system, even on your quite powerful machine. (... let's see ... that would be around 1200 convolutions to handle - the equivalent to 600 Vienna Suite Convolution Reverbs ... :-/ ). Maybe you should start with a smaller stereo test project first and see how far you can get.
... and regarding your last question: It's part of a surprise that it should be - well - surprising. 8-)
Regards,
/Dietz - Vienna Symphonic Library