Scarlatti and Bach can be great for the beginning pianist because sometimes their pieces can sound great even at the slow tempos beginners have to play them at!
As far as the triplets/appogiatura issues - yes, you are right, you need to nail down getting the triplets correct against the quaver. I've had miserable experiences myself trying to get ornaments down when the underlying rhythms between the hands aren't mastered.Practicing separate hands is also a good idea and when trying to get the triplets correct, use a metronome for the quavers, dividing the beat into three until it is second nature. You'll get there - I did!
The good news with the triplet is that it basically comes on the beat against the bass, so you don't have to be concerned with 2 against 3 relationships between the hands.