milah done by a katan

Shitas haRambam (Milah 2:1) is that a katan can do milah.  Where did the Rambam get this from?  R' Chaim Kanievski cites the gemara (A"Z 27) that tries to prove that a woman can do milah from the fact that Tziporah did milah.  The gemara rejects the proof and says maybe Tziprah asked someone else to do the milah, not that she did it herself.  Who else could she have asked who was Jewish, says R' Chaim, other than her other son Gershom, who was a katan?    

(Parenthetically, as for how a katan can have kavanah for the mitzvah [as the Minchas Chinuch asks], see the Chelkas Yoav 1:33 who writes that mitzvos tzerichos kavanah only applies when the mitzvah is a one time act, not when it is a 'peulah ha'nimsheches' like milah is.)

R' Ben Tzion Aba Shaul quotes a kashe that was asked to R' Chaim on this.  We learned 2 weeks ago that according to shitas haRambam all future descendants Keturah, not just her immediate children, are chayav in milah.  If so, maybe Tziporah asked one of the bnei Keturah to do the milah, not her own son?

R' Chaim answered by quoting a Mes Sofrim that says that when Avraham banished the bnei Keturah, they were imprisoned somewhere and could not escape, and so they could not have been present with Tziporah.  R' Ben Tzion Aba Shaul quotes gemaras that seem to not assume like that Midrash.

Based on the chakirah of R' Erlanger in his Birchas Avraham that we discussed, the kashe is not a kashe.  Yesh lachkor: is the pasuk that is mechayeiv the bnei Keturah in milah a new din, or an extension of the chiyuv milah given to Klal Yisrael?  If it is a new din, then the bnei Keturah cannot be mal someone from Klal Yisrael, as the chiyuv milah is not the same chiyuv, even if it is the same act.