Is there anything that must be skipped, such as viduy, in Selichot when a Baal brit, Mohel or Sandak are present just as it is done in viduy Shachrit or Mincha in such a case?
16: If there is a Bris, and it still hasn’t become light outside you certainly need to do Nefilat Apayim. There is an argument amongst the Acharonim if it already became light outside whether or not it should be said, but it seems per Halacha it should be said. And even the Sandak should say Tachanun and Nefilat Apayim in Selichot.
(סליחות המפורש החדש - page 3)
Footnote 38: אלף המגן ס״ק ל״ה בהגה״ה
You can infer the following regarding Selichot said on a fast day. According to O.C. 131:5 if there is a brit in shul, the congregation says Selichot but Tachanun is not said.
One could also "argue" that during Ellul & 10 Days of Repentance, Selichot are said prior to Shacharit, anyway. Thus, the "normal rules" of "exemptions" b/c of a brit that might, theoretically apply to Tachanun don't apply to the Selichot, in this case, anyway.