Do 1.6 Sierras have different sized first motion shafts from the 2.0, and therefore different clutch pilot bearings in the end of their respective crankshafts?

Yes and no! Some four speeders have a smaller 22mm x 20 spline shaft as opposed to the 25.4mm x 23 spline shaft but they use the same diameter spigot bearing in the crank.

all *sierras* had the same shaft though as they had the uprated 4 spd or type 9 5 spd, not the small 4 spd escort box

The early 1600 Sieras had smaller clutches though