I am not a lawyer. My only claim to scholarship is I know how to point and click and turn an occasional book page.
I have not read the opinion. But . . .
I would say, "no," they cannot be impeached for "citing international law and custom and not our Constitution be impeached and removed from the high court for delving outside of our Constituion."
From what I have heard and read, the reference is made as "dictum", as a judge's "expression of opinion on a point other than the precise issue involved in determining a case" (Webster's definition)
In other words, reference to outside facts and practices sheds light on the judge's reasoning and is not binding in any way.
We may not like it, but in this instance, I do not think Kennedy did something he could not do.
If he is allwoed to source foriegn laws customs and practices then MAYBE someday it WILL be an impeachable offense.......see my post no. 46.