Admiral James Kirk wrote:
Patrick is his mother's son as far as looks go. I'm about as scared of Patrick as I am any other skinny douche I pass on the street every day.
In other words, he'd be perfect for the part. The Terminator should look like someone who's not dangerous at all, if I were Skynet I would create a model that looks like a 12 year old sad orphan.
If you were Skynet and creating a terminator then that would be the smart thing to do. However if you were a director casting the lead villain in a movie that wouldn't be so smart.
Don't get me wrong a terminator who isn't built like a Sherman tank isn't a bad idea, since they are infiltrator units. However you still have to cast somebody intimidating in the part.
James Cameron originally cast Lance Henricksen. Henricksen with his odd, lined features and creepy eyes would have been extremely
intimidating as a Terminator. Patrick Schwarzenegger on the other hand is about as scary as your proverbial 12 year old orphan.
However the OP's suggestion for Kyle Reese, Ryan Gosling would be a better choice for the Terminator role. He's small and plain featured but as his performance in the film All Good Things
proved, he can be chilling. Even deadly. He's a perfect choice if you're casting a little guy as a Terminator.