Naw, pretty doubtful. It's probably just coincidence that a Hive Fleet stumbled across the Outsider. Or perhaps the Hive Mind was able to sense its presence somehow and had the Tyranids advance to within a certain distance before deciding that it was best to just leave the Outside alone in his Dyson Sphere.
There's really no sense in assuming that the Outsider is controlling the Tyranids. The Tyranids are a race that is very closely tied to the Warp, a realm which the C'tan find utterly incomprehensible. It's highly unlikely that any C'tan, let alone one as crazy as the Outsider would use such a medium to control their puppets. Besides, I'm sure the Outsider has his own Necron Legions somewhere - they're probably still inert on inactive Tomb Worlds throughout the galaxy, waiting for a signal from their leader to begin the Red Harvest.
This seems plausible yes. Good for you on coming up with such a cool theory.Maybe the Old Ones knew about the coming of the Hive Mind (probably feeling it through the Warp) and created the Krork not to fight the C'Tan and Necrons, but to fight off the Tyranid. If you think about it, the Orks are really the only race big and widespread enough to stop the Tyranid's progress. They aren't even going to necessarily kill the hive fleets, they just need to halt their momentum and tarpit for long enough that the Hive Mind decides that the milky way isn't worth the investment, and goes off to eat some other galaxy.
I'm not sure that the Tyranids are interested in giving up, though... in fact, I can't think of any circumstance where the Tyranids actively fled battle except after Hive Fleet Behemoth's *** kicking at the hands of the Ultramarines and Imperial Battle Fleet due to the largest ship in the Hive Fleet being destroyed. At least as far as the fight between the Orks and Hive Fleet Leviathan goes, I don't imagine that the Tyranids will ever stop trying to consume the Orks. There's far too much to gain by eventually overpowering the Orks to give up. Besides, I've always personally thought that there was a limit to just how big and nasty Orks would get if subjected to constant warfare. We know from reports from Armageddon that even extremely young Orks are equal in size to full grown Orks born on other worlds, as the bodies of the young Orks are influenced so heavily by the magnitude of the fighting. It has to stop somewhere though, or we'd eventually have baby Orks the size of Dreadnoughts fighting off dozens of Gaunts at a time.