Were Tyranids Sent By Old Ones?

I have a theory as: Remnants of Old Ones escaped out of Milky Way after War In Heavens. After that, they created Tyranids just as they created krorks in War of Heaven times (or maybe they were created before escape but didn't have chance to unleash them) and were sent by Old Ones to strip intelligent life off (no intelligent life, no chaos. Because chaos is too big to fight them alone) in Milky Way to prepare galaxy for their return. But this theory doesn't add up for Necrons. They know Necrons are out there and Tyranids can't consume them. So, why create and send them just for organics...? I would like to have your opinions.