Hi Harry,
Karel mentioned it already in his reply: MIR Pro's concept is based on the processing of single instruments or ensembles of (more or less identical) ones. Routing-wise you will be able to use mixed groups within one "Icon" in a MIR Venue, but you will lose most of the very specific features MIR uses for Vienna Instruments (... the detailed Directivity Profiles being the most relevant one, as well as convenient options like Natural Volume and tailor-made Character Presets).
While the results can (and hopefully will) still be pleasing, you will give away much of the realism achievable otherwise.
Kind regards,
/Dietz - Vienna Symphonic Library