Is it correct to say, "I am the first of the Muslims" in the opening supplication (Du'a al-Istiftah) of prayer, or is it more correct to say, "And I am among the Muslims"?

What is said when commencing prayer is not among the transmitted supplications (du'a), so it is better to omit it and restrict oneself to the transmitted opening supplications. And your saying, "and I am the first of the Muslims," which appears in the opening supplication, does not mean that you are the first Muslim, but rather its meaning is to hasten to surrender and submit to the commands of Allah the Almighty and to obey them.