This is how I did my CFI ratings. If single engine CFI add ons are not the typical way to do things in your area, make sure you have an examiner who is able to do it. Oddly, being qualified to do single engine CFI initial exams doesn't automatically qualify the examiner to do single engine CFI add-on exams. Nobody had done the MEI first here in a very long time, so none of the examiners in the state were qualified to do a single engine add-on exam. It was a hassle to set up and involved a pair of FAA inspectors observing the exam.
It had been 4 years since I did the commercial single engine but the manuevers came right back to me within an hour. Pay attention to coordination and left turning tendency. You may have gotten sloppy with that.
As I recall, the oral consisted of 3 items and I think they were a repeat of items from the multi engine exam. One of the FAA inspectors observing the exam was the same one who did my MEI. When the examiner asked a question, he chimed in that he was pretty confident I was going to answer it correctly because he had asked me the exact same question for the MEI. It was kindof weird actually.