Well Imperial Guard could work, considering that alot of the Imperium technology is very likely based off of Necrons. The Machine Cult supposedly worships the Void Dragon afterall.
The Tau idea sounds like it could work too. Though I don't really see the Necrons working with the Orks. The "Krork" (as they used to be called) were one of the races created by the Old Ones, and the Necrons hate everything the Old Ones created. That goes for Eldar too.