If you include: " /cast [pet:imp] Firebolt " in your macro, it won't make a difference what the pet is on.
It will be casting Firebolt at your current target, as you spam your macro.
And if you switch targets, it will switch with you.

Aggressive will probably get your pet killed.
Defensive shouldn't be much different from passive, in that the Firebolt line will have your pet attacking before the automatic attacks, in response to your warlocks attack, kicks in.

Still, not sure why the lock is that far back of the shammies.