Actually, the Necrons are already opposing the Tyranids. Szarekh, who used to be the Silent King in charge of the entire Necrontyr species, views the Tyranids as the greatest threat there is to the dream of his people regaining biological bodies (and thus their souls), so he is marshalling tombworlds against the Great Devourer. The whole "all necrons want to eradicate all life" thing has been tossed out the window by Games Workshop.
Indeed , ( as mentioned in my original post ) though without the command protocols the silent king has no control over his former empire and I doubt all the dynasties will heed the call to fight ( indeed he himself is probably a target of some vengefull lords as not all went to transference willingly ) .
Even with the Necrons , I doubt ( unless GW change the fluff ) that the hive fleets could be stopped .
Just leaving those that don't support the silent king to sit back a wait ( theyre good at that
) till theyre last ...... robot standing ;)