Does xingyi have the same principles of contradictory force as yiquan?I know yiquan came from xingyi but apart from the hands tearing cotton are there any other forms of contradictory force eg vertically etc

The way my teacher explained it to me is that the contradictory force is critical in developing power. In pi when your splitting hand falls the head slightly elevates. In zuan when the fist rises the body sinks slightly. In beng the important contradictory force is the horizontal rotation of the waist, not the up/down duality. I haven't really figured out how it works for pao but I think its more like beng. Heng you sink as the yang fist rises.

In Pao, it probably also involves the rotation of the waist... according to your principle of contradictory forces above.

Pao (and Heng) are both Ao Bu (opposite fist/foot)

... as opposed to pi/zuan, which are Shun Bu (same side fist/foot - which is why the vertical displacements you mention are important)
... and beng - which alternates aobu/shunbu - (which is why the waist/horizontal is so critical for this fist)

In Pao, as you step (from outside)into the centerline and your arm/fist extends, there is simultaneous rotation to the opposite side/foot.

This combination helps develop the power of Paoquan.

Heng, as you suggest, has a subtle sinking component... but perhaps more important is the arc of the fist (as opposed to the straightness of pi/zuan/beng/pao) - as it curves from the outside to the inside, combined with the waist rotation, of course.

... only my humble interpretation, hope it helps.