I am thinking of using this guide rail with a Makita plunge saw. Is it compatible?
Yes.I run the makita on the festool track and it works great.
When cutting your anti-splinter flush, where do you start the saw?
Put 1/2 the saw on the track at the Beginning. As you put the blade down to cut it will be enough to cut it all.
What are the size of the T tracks facing up and down? (width/height)
Depth is a hair under 9 mm and width 12 mm.