So I am not an expert, but my understanding of crosstype is the focus points ability to be sensitive to both vertical and horizontal contrasting lines, meaning that non-crosstype will have more difficulty to lock focus depending on the orientation of contrast in the subject.
I also thought that the AI Servo tracking has to do with algorithms and software insofar as the ability to track moving subjects. So theoretically, if the AI Servo on the 6D is capable (and there are some threads with interesting results on this), and the subject matter is sensitive to the focus point you're using, then whether its crosstype or not doesn't matter in that case.
Similarly, if you're not tracking moving subjects but are shooting a still scene, again depending on the subject matter in view of the focus point, a crosstype can generally be more likely to lock focus, if the subject matter contrast is perpendicular to the similarly placed non-crosstype focus point's sensitivity; otherwise either point would be able to lock focus.
So it seems to me that while crosstype points are more likely to lock focus on a wider variety of subject matter, this has nothing to do with the camera's tracking ability (although it can be helpful for subjects that the linear focus point cannot lock on to).
Just my thoughts. If I am wrong, just ignore me. Nothing to see here, move along