First of all, I would rather leave my earth caste with a hoarde of orks then with you Tyranids. The out come should be just about the same anyway.LIES! Silbern you must remove the Imperials! They are the bigger threat! you can always ally yourself with the... umm... Necrons and kill the tyranids later!
And leave the earth caste behind to build the fortress all by themselves so it will be done when you get back.
Don't worry everyone. All I'm going to say is the enemy of my enemy is my friend. So to place my "Carrion" plan in to effect I will probablly let the Tyranid take the planet. This campaign is getting interesting indeed.
I'll take some losses, understandable, but with the addition of Grey Knights to the Imperials means I will need to take a wait-and-see approach. Sooner or later the Tyranids will have to face the Imperials who are also trying to fight off the Necrons! Carrion indeed.