In my extended family some eat only chalav Yisroel and some use chalav akum. Can those who are chalav Yisroel only, use utensils that were used for chalav akum?
Even those who are stringent and avoid drinking chalav akum can be lenient concerning the use of utensils that were used with chalav akum.
Although the original prohibition of chalav akum applies to utensils, too (see Rema, Yoreh De’ah 115:1), it is possible that this does not apply today, because we know that our milk is free of non-kosher milk (due to regulation).
Although many rule that the prohibition on milk itself remains in place, this prohibition does not extend to foods that are formed from the milk, such as butter and cheese (see Rema 115:2; Minchas Yitzchak 9:25; Melamed Leho’il 36). By extension, the belios (taste) of such milk (which is known to not contain non-kosher milk) will not be prohibited. This ruling is given by the Tzemach Tzedek (Lubavich), no. 76.
In general, we find a basic distinction between a food item itself, and the belios from the item, as the Mishnah Berurah mentions (Biur Halachah end of 489). This distinction can certainly apply to chalav akum today, which is permitted according to some authorities (as the Iggros Moshe writes, though he, too, writes that one should not drink the milk itself wherever possible).
In addition, the vessels will usually not be benei yomo for such milk.
Therefore, although if a person is stringent it would not be considered a “mistaken chumra,” there is definitely room for leniency in utensils that have absorbed the taste of chalav akum. In a family environment, where being stringent can cause discomfort, leniency will have the upper hand.