I read all DCI specs and actually never found a clear word on minimum channel count requirements. Unfortunately, the recent edition of the DCI spec links to SMPTE specs very often now instead of giving detailed definitions in itself. I don't have the money to afford these specs ;-)

True, you're on the safe side only with a stereo or 5.1 channel setup and silent channel fills. 

DCI/SMPTE left aside, the servers may still all behave different depending on brand or software version.

Wolfgang?

- Carsten