The Slim in a nude state is shorter than a nude Narrow, but, depending on the costuming and all, putting a figure on a Slim can have no affect on height, if it's a figure with boots where the ball sockets plug in higher up than other figures, taller neck pieces, etc., the small heigh difference in nude state can be lost.
However, overally mass, I would lean towards Narrow or custom for T-1000, the shoulders don't seem as narrow as the Slim body.
It's really poor of HT not to let people know what body they'll be needing.