Is it ok to play free stroke? I'd play free stroke throughout the Sor's piece at least. Then, after it is confident enough I'd try to introduce rest stroke.
BTW, I just discovered that "Sight Reading for the Classical Guitar" by Robert Benedict is amazing, I started from book 1 from simple exercices. Very well organized and progressive.
Also, found this site http:// t h e f l u e n t g u i t a r i s t.com/all-exercises (remove whitespaces) - a very big collection of different music arranged by difficulty to practice guitar sight reading.