At Har Sinai the Jews became converts, for we learn Hilchos Geirus from Kabalas Hatorah as the Gemara in Krisos 9a says that all the male Jews who exited Egypt circumcised themselves at Mount Sinai as part of their conversion process. Being that the Jews re-accepted the Torah in Shushan-Kimu Mah Shekiblu Kvar, seemingly we can learn something about conversion in the Megillah.
The Gemara Kiddushin 70b says קשים גרים לישראל כספחת- Converts are difficult to the Jewish people like vinegar. Rashi explains this: שאינם זהירים במצות והרגילים אצלם נמשכים אצלם ולומדים מן מעשיהם
Rashi understood the statement as a critique of Converts, "for they are not zealous in their Mitzvah Observance, and other Jews are attracted to them and learn from their ways"
However, famously Tosfos 70b-71a quotes Rav Avraham the Convert who interprets the Gemara differently:
That on the contrary! Because converts are so learned and zealous in their mitzvah observance, they are detrimental to the Jewish people for G-d is reminded of the iniquities of the Jewish People when they do not do his will.
Presumably, in direct contrast to Rav Avraham the Convert quoted in Tosfos, Rashi assumes their lack of 'zehirus' in mitzvah observance comes from a lack of background and knowledge.
Are there any sources to support Rashi's interpretation?