Talk to your teacher. This question isn't really about the Scarlatti, specifically. You just need to build your technique concerning fast repeated notes, or simply getting a staccato out of them. Your teacher is the one that you need to ask. S/he will be able to see what you're doing wrong and correct it; something we can't do. However, no matter what, the answer is going to involve a huge amount of practice and drills. If you came here looking for a different answer, I'm sure you can get one, but it's not worth sh*t.
......or any other combination. By all means do not use one finger!
Case and point.